Maths required!

PRAISETHESUN

If the balance is paid off before the statement due date there is typically no interest to pay. If the sale takes longer than expected, or if the finance is treated as a cash advance or similar, that would change the situation, so it would be worthwhile considering this.

Terry_Towelling

Your biggest issue will be the way the transaction is processed. If it is processed as 'cash' you will pay interest on it from day 1 and you may be stung with a cash handling fee too.

Best to do a quick check of your account online to see how it comes through and then the forum will be able to more precisely target the answer for you.

Dobbibill

If the card was empty and it was processed as a purchase not a cash transaction then you would have up to xx days interest free grace period.
As you are carrying a balance, this is not the case and interest would be charged from the date of purchase until cleared in full. There will also be trailing interest the following month. To return to getting your interest free grace period, you have to pay two statements in a row in full.

omgimcrap

thank you all for your advice.
much appreciated.
Tim