[Graham]

Are the old Scalextric Mini Coopers allowed?

...wheel arches will have to be sanded back to 50mm of course

I would say yes if the wheel arches are sanded back to 50mm, and that if the wheels remain the same width as the main body. I believe that some 60's Minis did race with slightly modified wheel arches (but not as wide as these Scalex ones of course)