Thanks for the reply.
I've been doing some background reading since posting. It seems the
expected behaviour of an AP isn't defined after moving frequency - they
get the choice of waiting then possibly moving back if the band's clear,
or simply staying put. Either way can be problematic. This particular
one stays put on the low channel.
It does seem that whatever higher channel I pick, it eventually gets
bumped by radar detection, so I assume that everything above 48 is so
occupied for significant periods
I did see an ofcom document suggesting that bands just below 6GHz had
been deregulated in the UK; but I don't think my AP stretches that high
(it's a fritzbox). Would have been useful.