Many thanks to all of you for giving me your thoughts and experience. I've always said this site is excellent for the quick and useful advice from (most of) those who use it.
I'll do a bit more research - I see Kenko tubes appear quite similar to Canon in that they use metal where it matters.
I'm lucky to also have the 300mm f2.8 mk1 but couldn't carry it and the 500mm around at the same time, hence thoughts of extension tubes. (Why have one mid-life crisis when you can have two and buy a big white each time to cheer yourself up?
As a small thank you, have a slightly over-saturated shot of a Nuthatch taken with the 300mm (I'm not really a bird watcher but am told the Nuthatch is very rare this far north in the UK. There's bleedin' loads in my local woods).