Many thanks for your help.
The switch in the upgrade kit is a CRL switch but I've managed to find the dimensions on line and it will (probably!) fit in the factory routing which is 35mm deep. If not I have some very sharp chisels which should do the job.
Since asking my original question, I have found two or three excellent YouTube tutorials that cover how to wire up a Strat pick guard from original parts so, as long as I take my time, I should be OK.
Once again, many thanks.