I recently bought a Horstmann C27 Programmer and Digital thermostat. I
installed the C27 and it worked fine. I then installed the thermostat
and stupidly guessed which neutral wire to stick in and guessed the
wrong one - resulting in a pop and untimely death of my C27 programmer.

I then got the correct wiring for the digital thermostat and the
electritian who fixed it used a workaround on the backplate wiring so
that both hot water and heating switch on from the switch with the
expectation that I would move a couple of wires (I'm sure I've got this
right) and plug in my next door neighbour's heating clock temporarily
when I borrowed it.

Here's the problem. It works on his but not on mine, despite everything
else, incluing new thermostat, working from the switch! Anybody got any
ideas?