keith wrote:

I'm going to reuse a circuitbreaker position that used to power the bell
transformer, to provide a dedicated fused connection unit for a new alarm
system. The circuitbreaker is 6A, and I'd use a 3A fuse in the FCU, so in
theory I can use 1mm lighting cable.

But something is nagging me that I ought to use cable at least enough for a
13A socket, in case some future house owner swops the fuses for 13A? Is
that a best practice or am I being paranoid - what do the experts think?

You are being paranoid - 1mm^2 will be fine. If anyone changes the
purpose of the circuit again in the future it's their responsibility to
ensure that it's appropriately rated for the new use.

--
Andy