It's a hybrid and a charicture of sports presenters in general. There is a fair bit of David Coleman in the hyperbolic delivery and phrasing used on 'The Day Today' and I am sure Coogan simply expanded the notion that a lot of TV people really believe they are more important than their guests or the medium, to make the hideous creation that populated the chat show series. If you observe the Enfield / Whitehouse personnas Mike Smash and Dave Nice (even the names make me laugh) they work on a similar principle.
Coogan himself gave the same explanation as andy hughes when I saw him talking about it on some chat show a while back. A hybrid of presenters, no-one in particular. I reckon there's a good dose of Richard Madeley in there though.
don't know if any of you have seen this guy, but there's a chap on QVC who IS Alan Partridge. His name is Dale somethign or other and I swear he is SO like AP...uncanny. But yeah, I had also heard Coogans answer to this question...Andy is right.