Surely bad advice? Protecting a 29A rated cable with a 30A rated fuse
has to be bad karma. The 29A is max isn't it? And the 30A fuse could
take substantially more before it blows?

No, it is fine, provided there is ONE fused connection unit at the end (or
ONE single or double socket). It is no different from an unfused spur run in
2.5mm on a 30A radial/ring circuit. Overload protection in the 2.5mm spur is
from the FCU/plug top fuse. Short circuit and earth fault protection for the
cable is provided by having sufficiently low earth loop impedence. As the
majority of the circuit is in 6mm and there is only 3m of 2.5mm, this is
unlikely to be a problem.

The fuse/MCB rating will cover any cable that has that rating. The cable is
designed to stand substantial overload for much longer than the protective
device.

However, there is no need for this circuit to be 32A. If it only runs the
central heating and a shower pump, I'd replace the MCB with a 16A or even
lower. I see no reason to have higher MCB ratings than necessary.

Another alternative is to replace the junction box with a 13A FCU, if it is
accessible for changing fuses and checking terminals.

Christian.

P.S. being paranoid, I tend to run shower pumps off an RCD, even when
plumbed in plastic.