I don't *think* that this is an entirely stupid question, so here
goes. Background: running power out to shed, 50m of 4mm SWA, spurred
off the ring main. House earth is TN-S, planning on using a 100mA time
delay RCD FCU at the kitchen end and a two way CU with a 30mA RCD & 6A
& 16A MCBs. So far, so good.

Problem: I've tried the usual suspects (Screwfix, TLC, Toolstation
etc) and while the garage CU is widely available, I can't find a 100mA
time delay RCD FCU. I could get an enclosure and a two module RCD (TLC
do an MK one for some infeasibly large amount of money) but it's not
the ideal form factor for the location in the kitchen - Ideally I just
want to drop the RCD FCU into the box currently occupied by a fused
switch.

Any ideas, or am I looking for a product that nobody makes?

TIA
--
Rob