I tried to ask for that MattUK, specifically for a large one-time charge, but it was a no-go. In fact they specifically told me the same thing that theaxe reported: putting your account in credit will not impact the amount you can spend on the card.

I think the split transaction is the way to go. Just tell them how much to put on the card otherwise they'll automatically put the entire amount on and it will be declined.

Good luck and let us know how you get on! =)