we have found what apears to be leek moth caterpilliars on our leeks they are about 10mm long and of a cream colour

I wouldn't use any chemical spray on a food crop, it will leave a residue in the actual plant. If they are big enough to see you can simply put on a marigold glove and remove them with your fingers. Alternatively get some vegetable soap, or if you use a ecological washing up liquid use a table spoon of that diluted in a spray bottle and spray. Quickest way is gong to be removing them by hand though.