Say I owe £2000 on a credit card, and I have £200 cash to pay on it, but I also needed to spend, say, £60 on petrol. Would it be better to pay for that in cash and just put £140 on credit card, or is it best to put the full amount on credit card and pay with the card for the petrol. It's probably really obvious - but not to me!
It would be better to pay what you can on the credit card and use the car for the petrol. That would be paying off some interest, and the new use would not yet be accruing any interest.
If the £2000 is a running debt (ie more than one month) then it makes little or no difference which way you do it. On most cards, new purchases incur interest from day 1 if you aren't clearing the full balance each month.
Not owned a credit card for years and years, but it was my understanding if it is not paid off in full then interest accrues on the remaining. So clearing the card and starting afresh would be my suggested best option.