Martin Angove wrote:

Bear in mind too, all those who have been giving cable ratings for 6mm2
cable, that with rewireable fuses you have to derate everything to 0.725
times nominal (433-02-03) thus 6mm2 cable at method 1 which would
normally give 47A is only rated to 34A.

Not in this case: the 0.725 cable derating factor for circuits with
rewireable fuses only applies to circuits liable to be overloaded. A
shower circuit isn't liable to overload because you have a fixed load
resistance. Overload protection can be omitted [473-01-04(ii)] and the
433 group of regulations then doesn't apply. See also appendix 4,
section 6.3.

I'd be doubtful that a wrong rating of fuse caused the burning
originally described.

I'd say it can certainly contribute though. 35 / 30 is 17% over the In
of the fuse, leading to 36% excess power dissipation in the fuse
assembly (more in fact because the resistance of the fuse wire will
increase when it's hot). The temperature rise of the fuseholder parts
could then be, say, 40% more than was intended - probably enough to
contribute to the slow cooking.

As others have pointed out, this is far more
likely to be down to a loose connection somewhere. Putting a 40A
rewireable fuse in won't cure this and is likely to be (as I pointed out
above) well outside the rating of the cable, even at the most lenient
installation method. Ignore what I've said if you've actually installed
a cartridge fuse (looks like a big plug fuse and says BS1361 on it)
rather than a rewireable.

I beg to disagree. As I said in reply to Grunff, increasing the fuse
rating won't affect how hot the cable gets. Yes, the cable installation
conditions should be checked as part of the process of putting this
right - but in relation to the actual load current (the design current
of the circuit Ib, modified by any grouping and/or ambient temperature
correction factors that apply) not the fuse rating. The important thing
is to check the earth fault loop impedance to ensure that, with the
larger fuse, 5 s disconnection time is still met, and for adiabatic
compliance of the CPC.

[Screwfix CU] though as this comes with a 50A MCB you'll either
need to change to 10mm2 cable or swap the MCB for a lower-rated one.

I'd swap the MCB anyway, but again, in a circuit not liable to overload,
the cable rating required can be determined by the load current, not the
protective device rating. There may be no need to replace the 6mm^2,
provided you're prepared to do some sums.

--
Andy