Thanks for the reply,
I have not measured the hole in the stock plate. It is not possible to get a caliper or other measuring tool in there to do so. However, I do like the idea of cutting a piece of cardboard precisely the size of the hat on the new Tstat, gluing it to a stick and see if it goes into the old hole. It would be bad if the little cardboard disc fell inside though!
I'll try it but my assumption is that it won't fit.