Thanks for the suggestion. I wish I had a local joiner!
I have ordered the biggest bit I can find with an 8mm shank. (It's a
1/4" shank - I hope my router will be able to grip it sufficiently). The bit is 25.4mm dia - not as big as I would have liked, but it may suffice. It was affordable anyway, at £16.96 so that's a plus.JD