This year sees the final National commentary for the man who has called every running since the race was first televised in 1960 - including the One That Didn't Get Away in 1993. He has been covering the race as journalist or broadcaster (or both) since 1947 - which, subtracting the 1952 renewal when the redoubtable Mrs Topham brought in her own team of commentators, makes 1997 O'Sullevan's 50th National. But it is still long odds-on that asking him down to the bar for a few valedictory noggins will be met with a gracious refusal.
Behind that closed door 79-year-old O'Sullevan will be putting the finishing touches to homework which began in earnest six weeks ago when the original entry of 103 horses was reduced to 86 possible runners. At that stage he started to familiarise himself with the colours, to unearth facts and figures, and to embed in his mind a mass of information which, come Friday, will have been distilled on to a cardboard folder bearing the appropriate details of every participating horse and jockey.
O'Sullevan's voice - described by Russell Davies as "perhaps the only hectic drawl in captivity" - has become as much a part of our collective memory of the National as Red Rum or Bob Champion.
The moment on the second circuit when John Hanmer brings the horses across the Melling Road and declares "over to Peter O'Sullevan" is a signal for the entire nation to shift to the edge of its seat.
But it is the ringing phrases which take the leaders up the run-in, round the Elbow and towards the winning post which stick in the memory. In 1973 the appalling truth dawned that Crisp was about to be caught by Red Rum: "He's beginning to lose concentration - he's been out there on his own for so long."
Four years later Rummy scampered up the run-in for an unprecedented third victory: "He's coming up to the line, to win it like a fresh horse in great style. It's hats off and a tremendous reception - you've never heard one like it at Liverpool." Those phrases linger in the litany of the sport as securely as "They think it's all over - it is now".
Now that litany is approaching its Amen. By teatime on Saturday those familiar tones will themselves have come round the Elbow for the last time. There are more calls to come -a final Royal Ascot, a final Glorious Goodwood - before he hangs up the microphone, but it is on Grand National days to come that his absence will be most keenly felt.
Aintree is making sure that the O'Sullevan legacy is permanently acknowledged. Before racing on Saturday the Princess Royal will unveil a bronze bust of O'Sullevan which will beam benevolently down on the paddock. "I hope it won't frighten the horses," says its subject.
And coinciding with the final National comes a book of tributes. With contributions from the Queen, the Queen Mother, the president of Ireland, a phalanx of celebrities and racing professionals, and the "ordinary" punters who have written him fan letters, Coming to the Last illustrates the extraordinary range of O'Sullevan's following.
From those tuned in at Buckingham Palace or Clarence House to the once- a-year 50p each-way punter coming in from the garden to watch the race, they'll all be bidding him a fond adieu on Saturday.
Coming to the Last: A Tribute to Peter O'Sullevan is published by Partridge Press on 3 April.
1949: Russian Hero
Peter O'Sullevan's first Grand National broadcast, for BBC Radio
The senior commentator was Raymond Glendenning, and I was stationed down by the first. In those days you were expected to commentate standing on the ground, telling the story as horses fell at your feet - Russian Hero won this year.You could get no perspective as you could get no point of elevation. But at that time there happened to be, near the first fence, a little latrine with a sloping corrugated iron roof. Having persuaded an indulgent engineer to extend leads for both the hand-held lip microphone and the headset, I clambered up. Holding the microphone with one hand and race glasses with the other made for a very precarious position. In order to get any purchase on the iron roof I had to take off my shoes and socks. Clive Graham, assisting me as race reader, likewise removed his shoes, though he wouldn't go as far as to take his socks off. Peter Dimmock, the long-time head of outside broadcasts at the BBC, still insists that I never stopped complaining about facilities, and I wouldn't deny it for a moment.
The year Devon Loch slipped up on the run-in, fifty yards from the line, handing the race to ESB.
I was stationed at the 12th fence, which is the 28th, three from home, on the second circuit. Again there was no custom-built vantage point - not even a handy latrine - so the key issue was what sort of van the engineer had, whether he could accommodate me on his roof, or failing that, bonnet. In 1956 it was the bonnet which gave me at least a little elevation from which to call Devon Loch over the third last in the lead and full of running. As the leaders made their way back towards the stands, and were swallowed up by the crowd, I could follow Devon Loch's progress towards certain victory watching the jockeys' caps above the heads of the throng - the Queen Mother's black with gold tassel being pursued vainly by the green of Mrs Leonard Carver, owner of ESB. All of a sudden the black cap disappeared from view. That something sensational had happened was confirmed by Raymond Glendenning's commentary - and in an instant I had freed myself of headset and microphone and was racing back to the grandstand to get the story.
1960: Merryman II
The first televised National was always going to be fraught with anxiety. I spent most of the first day beefing to my producer about the absurd inadequacy of the monitor, an ancient set rented from a local electrical shop. As I was still grousing about it the second day, on National day itself the electrical shop was persuaded to send a technician to attempt to adjust the monitor, which was next to useless in daylight. He was no more skilled in twiddling knobs than anyone and eventually he shrugged his shoulders and gave up. On big race day everything came right. Merryman II had been a marvellous hunter chaser, and I'd advised his owner Winifred Wallace to send him to the trainer Neville Crump. Merryman was ridden by Gerry Scott, who rode with one arm immobilised by strapping as he was recovering from a broken collar bone. I'd tipped the horse in my Express column and I'd had a wager on him back in December 1959. During a night with the brandy bottle at Nottingham, the bookmaker Jim Bailey laid me pounds 1,000 to pounds 28 Merryman for the National, plus a tenner at 5-1 for the horse to complete the first circuit. Since Merryman started 13-2 favourite, that was a satisfactory transaction.
1977: Red Rum
One of the great things about Red Rum, which maybe has been forgotten a little, is that apart from his own achievement, he was instrumental in helping save the National at a time when its continued existence was seriously in doubt. There was so much focus upon him that the idea of the National being lost to posterity was greatly reduced. It was a privilege to call him home that day for his unprecedented third victory - maybe my most significant call. In the history of the race, Red Rum's timing was perfect.
For me the 1981 running encapsulated more than any other in the modern era, the enduring Corinthian spirit of the race. Here you had in Josh Gifford's Aldaniti a winning horse who had been a serious invalid and in Bob Champion a rider who had overcome cancer - and his nearest pursuer was the 54-year-old amateur rider John Thorne partnering his old hunter chaser, Spartan Missile. The emotional success of Aldaniti, who passed away last week, and Champion illustrated what makes the National such a very special race.
1993: Race void
The race that never was. Won by Esha Ness, but then declared void because several jockeys did not see the false-start flag.
There was a demonstration at the first fence, and a delay because the demonstrators had to be removed before the race could start. It was an atrocious day; the rain was horizontal and there was a very cold wind, which made the horses very agitated. Before the start I was constantly appealing to my producer over the talkback to be allowed to indicate what was holding things up, but he was getting instructions from London that they didn't want to give the oxygen of publicity to the demonstrators, so I should ignore it. That decision was understandable, but at my end it was frustrating not to be able to tell viewers what the delay was all about. The majority of runners set off following the second false start, and once the field was out of my sector as they reached the first fence I again asked the producer whether I could explain, when they came back into my sector, what was at the root of this shambles. But he insisted: "Sorry, Peter - just carry on." And so I did. I hope that day will remain unique.