The sombre character of his favoured reading matter, Nostradamus and the Book of Revelations, has done nothing to dilute his sense of fun. "There's a big war coming", Mortimer observes apocalyptically, "It's gonna go off between the Muslims and Christians". He pauses before adding, almost as an afterthought "and I come from Walthamstow, which is not a good place to be when that happens". East 17's identity is tightly bound up with the previously obscure district of outer East London whose postcode gave them their name, but Tony Mortimer no longer lives there. He's moved, with his girlfriend Tracey and their two children Ocean and Atlanta, to upmarket Chadwell Heath. He didn't always live in Walthamstow before either. He was born in Stepney, and his family headed gradually North East, finally moving from Hackney to E17 when he was nine. What did Walthamstow feel like when he first got there? "It felt easier: you didn't have to have so many fights - in Hackney I used to have two or three fights a week in the playground".
All the schools Tony ever went to have since been closed or amalgamated - and he often gets "what's that feeling?... Nostalgic" when he drives past the places where they used to be. East 17's clearly defined sense of place might be a product of dislocation as much as insularity.
The Mortimer family were "First generation cockney Irish". Tony still speaks with an Irish accent at home sometimes, but not when he's out and about. His mum was a cleaner and his dad was a carpenter turned site manager. Both his parents were musical. His mum was into Irish dancing and his dad loved country and western - Tony launches into a quick chorus of "You picked a fine time to leave me Lucille" to commemorate this fact - and at a tender age he was exposed to Elvis, Bobby Vee even Val Doonican.
His own main musical love was hip-hop, which came from America in the eighties - just as rock and roll had done three decades before - to delight adolescents and annoy their parents in equal measure. Tony and his friends were forever looking for a place to lay down their little bit of lino. "From fourteen upwards, I was the best dancer in the school", Tony remembers cockily, "I would start dancing in school discos and a crowd would gather round... That was when I first got people looking at me". Presumably - on the basis of what's happened since - he didn't mind? "I loved it".
Did American hip hop culture seem very foreign and exotic to him when he first experienced it? "It just seemed like something that was really natural and crudely beautiful: a way of expressing how you feel that a lot of us could relate to". Tony and his friends went to terrible breakdance movies like Beat Street - with their baseball caps on backwards, and his already well-established fondness for writing poetry fed naturally into a yen for the arcane squiggles of the graffiti tagger. "You can say whatever you want and people can't read it", he says approvingly. What sort of things would he write? "I don't know, 'Bollocks' maybe, or 'Love is alive'". This seems as neat an illustration as any of East 17's unique blend of street-smart aggression and earnest spirituality. Tony was in a gang once: "There were about fifty of us, aged from fourteen to about thirty. We never did anyone any harm - you'd just get kickings off people that were older than you". Nonetheless, East 17's cockney roughneck aspect is the only part of their image he's not happy with. "That came more from the marketing people... we ate pie and mash because that's all we knew - we thought everyone did what we did".
It still must have come as a shock a couple of years later, when Blur started to appropriate the greyhound stadium imagery East 17 had made their own. "It was just kind of 'think of something new fellas: we've done it already'. We used to hang out at the back of Walthamstow dog track and believe me, you wouldn't see Blur round there".
Long after debate has died down about the relative merits of Blur and Oasis, duels will still be fought on the East 17 vs Take That issue. Tony Mortimer's commendable insistence on saying exactly what is on his mind whatever the subject under discussion causes him to be less tight-lipped on this question than might be anticipated. "They had three singles out that got nowhere" - he observes, correctly enough, of the lissome Mancunians, before continuing in more controversial vein, "then all of a sudden they started copying the way we dressed and now they're more famous than us... they nicked us!"
Last year's well-deserved Christmas number one (and subsequent Ivor Novello Award nomination) with the classic "Stay Another Day" should have calmed E17's anxieties about being the poor relations of British teenbeat. And their global sales, including six number one singles in Israel ("If Israel's conscious of you," Tony once noted ecstatically, "then God's conscious of you"), far outstrip their Northern rivals'. But Mortimer is galled that the comparison should be made at all. In view of the satisfying directness with which East 17 engage with the shifting shapes of contemporary dance music - house and rap at first, then swingbeat and even jungle - he probably has a point.
East 17 are managed by Tom Watkins - the same tireless, acid-tongued Svengali who gave us Bros - so it is not surprising that people have tended to over-estimate the extent to which the group were manufactured, but there is no doubt that this is Mortimer's group. It is he who writes the songs, he who moulded his long-time friends John, Terry and mischievous, honey-voiced Brian Harvey into a convincing foursome, and he whose guiding hand on the collective tiller led TV comedians Lee & Herring to speculate cruelly on how many members of East 17 it might take to change a light-bulb (Four: "One to have the idea of changing the light bulb, one to change it, and two to stand behind them making handshapes and knowing in their hearts that their days are numbered).
With their third album now complete , and sounding more sophisticated and tuneful than ever, there is no sign of Tony knocking the group on the head just yet. Like all the best pop stars, he is an authentic one-off. There is an openness, even a vulnerability about him that is truly touching: he once told Smash Hits that his greatest fear was "being laughed at", and East 17's biggest hit Stay Another Day was partly inspired by the suicide of his elder brother. But he still retains the power to surprise.
Ever since the impeccably salacious "Deep" - "You lick the knife and I'll butter the toast" indeed - Mortimer has shown himself very much at ease with sensuality, and seems to fearlessly give voice to feelings men are traditionally expected to be shy about. Has he never had a problem with expressing himself in this area? "Not since I found out that I've got a big dick". Up All Night -(London CD/Tape) is out tomorrow. East 17 play Glasgow SECC, 1 Dec and the Royal Albert Hall, 8-10 Dec.