It is a sweet image. The highly paid professional playing fantasy football with his children in the manner of a million other less gifted dads up and down the country. All wearing the shirts of their favourite team. Recalling this moment last week, Merson made no attempt to play down its naive appeal.
"The kids said 'Put your Arsenal top on', so we all had our Arsenal kit on. And I'm like a kid when I'm out in the garden with the kids. I get involved, I get muddy and everything. I still do now, and I'll still wear my kit with them."
Merson wears his Arsenal shirt in slightly more organised games these days, an outcome that was by no means certain during that birthday kickabout. His revelations about a gambling habit that incurred six-figure debts, an inability to stop himself getting legless after the first lager and a growing need for cocaine made his the most sensational account of the life of a modern professional footballer.
His return to the game owes much to the support of his family, his club and the Football Association. Most of all, though, it has been fashioned by Merson's own remarkable resolve to face his addictions and try to conquer them. Rock Bottom, which tells his story and in particular details the harrowing sessions at a Southampton treatment centre, is a book of startling candour and brutal self-assessment.
A year after its publication, Merson has just competed his best season with Arsenal, one that culminates with his testimonial on Wednesday night. But the question you most want answered is whether he has persisted with the meetings - be they Gamblers' Anonymous, Narcotics' Anonymous or Alcoholics' Anonymous - that seemed to be such a crucial part of his rehabilitation.
The answer is yes, but it comes with a characteristic dose of self-criticism. There are still bad days.
When was the last one?
"Last week. The whole week virtually."
Do you know what triggers them?
"Yeah, when I stop going to meetings. I do know where I'm going wrong. Sometimes I think I won't go to a meeting tonight, and it just slides down. And then I get into the meetings again, and I'm fine. It's easy to turn round and say: 'I won't go tonight, there's a good film on.' That's when I struggle. Like Lorraine [his wife] says, there's something like 164 hours in a week, what's three hours out of that time? Three meetings. So there's no excuse. There ain't an excuse. When it rained, I went to the pub."
Draped on an armchair in the Mersons' sitting room, he is clearly as firmly rehabilitated into domestic life as he is professionally. Around him there are the unmistakable rhythms of a happy family. (One thing that was true even in the worst times is that he has always been devoted to his wife Lorraine and their three children.) Merson is relaxed but attentive as he responds to questions that many would find intrusive. Nothing is off limits, but the abiding impression is of a rigorous honesty as he assesses his own failings.
"No one forced me to take drugs, no one made me drink and gamble," he says. "That's the way it is. People look back on things and say that could have been a turning point, but I had everything. A lovely wife. Lovely kids. Big house. Cars. Great wages. I played for the best club in the country, by far. The fans liked me. And I done that."
The self-criticism is remorseless. "Do you think you're hard on yourself?" I asked at one point.
"My counsellor says too hard."
Physically Merson is taller than you expect, and unrecognisably thinner than the player who sported such a beer-gut that his team-mates used to tease him about being permanently pregnant.
Those were the days when "doing a Merse" was a drinking man's celebration involving the mimed downing of repeated pints and accompanied by a grin for which the word inane is barely adequate.
Photographs of this make Merson cringe now, but his assessment as to why he revelled in a bad boy image is revealing.
"I didn't make the character, the papers made the character. I lived up to it. Silly me lived up to it. I thought: 'I haven't been in trouble for a while, so I don't want to lose that tag.'
"But I was just a normal person really. You see, I didn't get it in my head that I was just a professional footballer for Arsenal Football Club. I was just a normal working -class man, coming from a working-class family, and what I didn't grab hold of was that I had to be different now.
"But that's why the fans liked me so much. 'Cos I am one of them. If they were in one pub down the road, and there was a wine bar up that road, I'd be in the pub with the fans. That was me."
He pauses for a moment, acknowledging the irony of what he has just said. "Now I can't do that, but then I didn't grab hold of what I should be."
A lot earlier in life Merson didn't grab hold of what it was he could be. The son of a coalman in west London, he was an outstanding young footballer, but not a particularly happy child. There were times when he did not want to play at all.
"I don't know why. People used to say how good you were, and I couldn't be bothered to get out of bed to play. You know I could easily have fell into the other side, you know football or dossing. Well not dossing, hanging around. It could easily have been like that.
"If it wasn't for meeting Lorraine really, settling with a steady girlfriend, it could easily have gone away. When you see your friends going out every Friday night, and earning two hundred quid at the building site, and you're earning twenty-five pounds at Arsenal, and you have to stay in every Thursday, Friday, you know it is hard. People reading will think, well how's that hard, playing football. But when you're young, your mind thinks completely different."
Even when Merson had made it as a highly-paid professional, there were still plenty of matches that he did not enjoy. A gambler since his teens, the addiction took a grip in his mid-twenties, to the extent that he became so preoccupied with his losses that it had an impact on his performance. His body, fuelled by constant lager binges and, eventually, by the recreational use of cocaine (though neither in the build-up to matches), was also letting the side down.
The nadir on the field came in a game against Brondby at Highbury in November 1994. "I was knackered after two minutes. With the worries of everything else my mind was gone. Yet I could have scored three or four goals. Not scoring, but us going through, probably done me a favour. If I'd have scored everything would have been all right that night."
Even that was not the lowest point, though. It came eight days later.
"It was the last time I took drugs. I was out on a Friday night, and it was about eleven o'clock. Lorraine had found out everything about my gambling debts and all that. I was with a friend and she rung up and said come home, we can sort things out. I was high on drugs and drink and I turned round and said I can't. And my little boy come on the phone, Charlie, and he was crying and he said: 'Come home, Daddy'. I couldn't put the drink and drugs down and I didn't go home until six o'clock, or whatever. I always remember that when we have a silence in the room in my meetings. I always remember that still, that: 'Come home, Daddy'. I don't want that again. I don't want that."
If Paul Merson's story has struck a chord with the public, the same seems true of his fellow professionals. Certainly the turn-out for his testimonial reads like a roll-call of the game's great and the good. Make that great and the very good.
Paul Gascoigne (who helped pay his own insurance), and Glenn Hoddle will wear Arsenal shirts, with Merson's all-time hero Ray Wilkins, Matt Le Tissier, Ruud Gullit and Andrei Kanchelskis among the opposition. Presumably the Highbury faithful will be issued with sedatives in anticipation of the excessive skill level.
For Merson many things have changed since he returned to an emotional ovation against Milan 15 months ago, among them his attitude to playing. "I've played every game this season, which is a feat. I've never done that in the 10 years I've been with the club. The consistency's there. It's not like one good game and six absolute nightmares."
Those nightmares were the reason he was regularly dropped by George Graham, Bruce Rioch's predecessor as Arsenal manager. Merson retains a lasting gratitude to Graham, and especially his support when the drugs, booze and gambling story hit the front pages, but there is little doubt that in playing terms Rioch's more sympathetic style of management has reaped a reward.
"The boss gives me confidence, saying: 'Get the ball, get the ball, get the ball'. And you think: 'I'm important here.' I'm not a person who needs kicking up the backside all the time."
Now 28, he would like the relationship with Arsenal to continue beyond his existing contract, which expires in two years' time, preferably for the rest of his career. But wherever he plays, Paul Merson's attitude will be the same.
"It's like a second chance. I've been given a second chance to enjoy it, and show everything you've got. Nowadays if it doesn't come off I'll go and look for the ball. Two or three years ago, if it's going wrong I could easily, sort of, drift out of the game, and not get involved.
"On Saturday, I had a couple of bad, bad touches against Blackburn, and then I chased and wanted the ball. Just to get a touch as quick as possible again, and get involved. A couple of years ago I'd have just drifted out to the wing, and if I got it I got it, but I wouldn't chase it. That's another thing I like about myself now, I will get involved and I won't hide. I do enjoy it. I should enjoy it."