Just to round up this post... Electrician has been this morning and was as dumbfounded as me. Turns out it was the PIR. He had a (different) one on the van and, after swapping it over, it now all works fine! He still doesn't know why it happened and has never seen it before!
Just to confirm, I did draw it wrong! Should have taken a photo rather than rely on my memory!! (Supply N goes into RCD...)
Anyway, sent a picture to my electrician who seems to think it should be working absolutely fine. So he’s popping in tomorrow.
Can anyone tell me what's going wrong here please? I have a feed into my shed with a simple little "garage consumer unit" on the wall... The type you get from Screwfix etc. There are 4 sockets running out of the 32A MCB (not shown on my drawing) and a single pendant light hanging in the middle...