Kev
From memory the garage check or ask for outstanding details of the amount outstanding to creation. This information will be entered on the purchase form for your new car. They the take off the trade in value for the new car. Your current creation account would be paid off, leaving a credit balance and this would be used with the new finance arrangement to buy the new car. The end result will be that you end up with a new finance agreement for a higher amount.
So first obtain trade in value, then approach creation, to see if they can replace with new loan for your new car. You could obtain finance elsewhere to settle the creation account and have a new loan. The only issues with this are, will creation allow you to end finance without any penalty fees/interest and can you obtain finance elsewhere anyway.
Garages come across this situation on a daily basis, so they will explain what happens. Ask for trade in value, but don't put down a deposit until you know you can definately finance. If you failed to get additional finance, the garage could be slow in refunding any deposit and could retain an amount to cover admin costs. You could ask the garage for a quote for the loan. Some garages can provide competitive loans, so may be cheaper, but as I said be careful about any charges that creation might make if you ended finance arrangements with them, before you decide.
Huckster