You, Sir, are a scholar and a gentleman.
One blown fuse located.
Many thanks for your help.
I shall now go away and give myself a good talking to! I really ought to have thought about a fuse!
Good day, folks.
I have a Paragon E alarm with four very old PIRs that have been painted over by several generations of decorators. Other than that, it is (OK, was) still in perfect working order.
I decided to change the old PIRs which look like this
The board inside looks like this...