Ian Stirling wrote:

For the moment, I think I'm paralleling them all, and sticking on a 16A
MCB - with the joins to the 2.5mm outside, where them going on fire
would at most destroy the buried conduit.

I don't need that much power at the moment - the main reason for putting
them all in initially was to reduce voltage drop - to be able to run a
3Kw kiln up there, and to hit it operating temperature spec, rather than
sort of wandering up there.

In that case it all gets much easier - it was the mention of a 40 A fuse
that started all the problems. If you can set the design current at 30
A or less then use just a 30 or 32 A fuse or MCB at the supply end and
hard-parallel the three cables. By analogy with the standard ring
circuit you know that the cables, including their CPCs, will be
adequately protected.

Whatever you do, do not put fuses in any neutrals - that would violate
one of the fundamental safety principles [Regs. 131-13-01 and -02].
(CBs in neutrals are only allowed if they are linked, so as to break all
associated phase conductors.

--
Andy