Student or Learner
(BBC London; Robert Elms; Elms: Barry Mason, Antiques & Hackney 18th April 2012)
Did he really place his accent by hearing that much (or, to be more precise, that little)?
(And one small additional question if I may - would you be so kind to tell me what exactly he says after '... that you still a northern boy at heart but you've [...]?)
We don't know how much he's heard. Presumably, he's heard Barry Mason before. (I imagine the preparation a radio presenter does before interviewing a singer involves listening to his records) On the basis only of the words 'Thank you for having me', placing the accent would be quite a feat, especially as the northern speaker has 'been here* a long time, haven't you' (the bit you missed at the end). In the years he's spent in London, his vowels have changed. He even has an /æ/ in 'Thank', a long way from the [ɑ] he'd've used in his home town of Chorley (Lancashire).
*As the interview was on BBC Radio London, 'here' obviously means London.
But generally, some people can tell a vast amount from a very small amount of data. The character Professor Higgins (based on the real life phonetician Henry Sweet - Wikipedia, the free encyclopedia ) in Shaw's Pygmalion (better known in its reincarnation as the musical My Fair Lady) was known for being to place you on the road where you lived (in a small part of London). More recently, I heard the actor Robert Hardy, in a radio interview, recalling his teacher at Oxford (J.R.R. Tolkien) going round his History of English class and relating a particular feature of the local accent to an aspect of that story - quite a 'party piece'! (I think the interview was on the programme Desert Island Discs - which is all archived somewhere on the BBC website, so if you're** interested....
** Incidentally, your transcription should have had 'you're', not 'you'.