Then CC-BY-SA is going to produce some truly moronic results under UK law.

Copyright on sound recordings in the UK lasts 50 years. So I take a 51
year old bit of music and use it as a backing for a film. Then I try
to release it under CC-By-SA

It is now a derative of that bit of music but copyright on films lasts
rather longer than 50 years in the uk so the copyright situation that
results is rather odd.

I don't see what's odd about that. It's a work, copyrighted by you, under CC-BY-SA, and based on a public domain work. It's no different than the same situation would have been if the music had been not 51 but 300 years old. And little different than the same situation where you had not put it under CC-BY-SA.