Cramped because I think the accessories are flat plate and the boxes look like 25mm.
Now I know what that means

OK I took the unit off, couldn't see anything obviously wrong, so did what I do with computers, cars etc - swapped the two units round to see if the fault moved or stayed. It moved, and it's really obvious now that the - what's it called - FSC - is faulty, as it still spits and crackles on a different set of wires, and the bottom one is fine
BUT as you've spotted, there simply isn't any way to get it fully into the socket. The utter, utter wakner who has butchered everything to do with every aspect of this house has worked his magic here too - he works for the council housing maintenance department, has obviously nicked whatever wasn't bolted down, then attempted to fit it.
He's found that it doesn't fit, so has just left it hanging out of the wall
So off to Screwfix now I reckon, to try and get a couple of units with a bit more space behind them...
Take one out and put it in the terminal next door to it.
The middle switch should be an intermediate switch.
OH! By the way, I got an intermediate switch, moved the wire in the other one as you suggested, and the hall lights now work as intended - thanks


