‘95 4.6 HSE - with the ignition on, there is more often than not a rapid, constant relay clicking noise from under the driver’s seat - I think it stops when the engine starts, but the hole in the back box may be masking it; it also starts when the seat backrest button is pressed. Pulling fuse 10 for the seats has stopped it for the time being - before I take it to bits, has anyone encountered this before? Are the seat relays separate units?

Yes, there are two power relays under each seat for non-memory seats. Not had a lot to do with electric seats but I do recall one that would blow the fuse if the backrest button was pressed.....

Thanks. I have seat memory - assume that means there’s an outstation box under there for it.

Yes, there's a seat outstation under there. Same left and right if you want to try swapping them over to see if the fault moves. Part number is AMR3359, plenty on eBay.