Bad credit rating - how?

[jno]

jno jnr wants to borrow more money on his mortgage but has been startled to find out from his building society that he's got a bad credit rating. He shouldn't have; all his payments are made on time, by direct debit. He's never had overdue or unpaid debts. He's gone off on a business trip worried sick, so I can't get any more details that that. I suggested checking with Experian but apparently he did this a while ago (no problems then) but worries that if he does it too often it will be seen as a bad sign in itself.

Anybody know how this might have happened and what can be done about it?

[jno]

He's got a mortgage, with his gf. Not certain what the problem was, but it may be that his current flat has different addresses for gas, water, etc ("Flat 2", "76A", "top floor" etc) and apparently doesn't exist at all as far as the post office is concerned.

Thanks for all advice.