Originally Posted by
KARFA
In terms of part using a £25 3V card, that's perfectly fine. The other £5 remians on the card to be used for anything else. You can also check the balance remaining on any card on the 3V website - it update almost instantaneously so easy to keep track of remaining balance.
and if you pay before your bill is generated they adjust your DD accordingly so that you pay less (or nothing if in credit). Unfortunately I've just switched to Virgin Media (for the broadband not the Tesco points !) and they don't seem to do amend your DD