[Graham]

Graham, if I want to use a short can motor in a 'P construction' chassis that has a hole for a long can, can I put ballast in the space or has that got to be plastic or an adaptor?

e.g. a Ninco chassis with a long can hole but using a short can motor

My view is that there is a difference between a M class car where the ballast must not be used as a substitute for any "standard" chassis part. And that would include a substitute for any part of a standard or replica motor mount adaptor, when changing from long can to short can motors. The adaptor should fit into the original motor mount position within the chassis as intended.
However for a P class car with adjustable motor pod you can use ballast within the chassis or motor mount as long as it is" fixed in place" and is not located lower than the rest of the chassis.