iliketosleep - I think you will find he was using the 'your mum' as a separate insult, in the Kentish style...as in
'oi, you're a fucker'
'yeah, well, yer mum'
It is a shortened version of an insult directed at the persons mother, for example, 'yer mums so fat she makes jabba the hut look anorexic'
They can't be bothered to think of anything witty so it just becomes 'yer mum'.
hope that helps 