This might be a stupid question...but....is there a way of working out what length of chain i would need for my BEC if i didnt have the old chain?

Piece of string.

Use the gearcommander on-line calculator.

Gear Commander

Load up a suitable bike and adjust the custom sprocket sizes then input a few chain lengths (even number of links!) and check the sprocket centre distances returned.

Check your current sprocket centre distance against the calculator's results and pick a chain length to suit your range of adjustment.