Cold call on my home number, terrible accent - but knew my full name,
address etc (and I'm not on the electoral roll) - claimed it was about
a reward voucher scheme for prompt credit card payment (ok, so no way
am I giving out personal information to these people - and card
companies *want* you to pay late).

Endless waffle about the benefits, basically hotel vouchers, but
willing to go off-script and answer questions - well, apart from "Why
don't you just post the stuff to me then, rather than give me all this
waffle?"

Anyway - eventually we get round to the £99 administrative charge, and
I say ta-ra.

Seems to be a relatively new one, though police aware in other areas:
http://www.psni.police.uk/pr_holiday_scam_040809

If the caller had better spoken English, I could imagine someone might
in a distracted moment not realise the call isn't from their credit
card company.