Thanks Mick
Still breathing - half an hour later, so it seems all is well.
Won't be fiddling with it again until the boiler gets replaced, so hopefully my lessons learnt can be quickly forgotten.
Thanks GasSafeMan/Paul... it took me a while to try and work out which were the original COM and ON wires from before (the old timer more or less disintegrated on removing it)...and one blown fuse later, managed to sort it.
It was the Yellow and Black wires. Now working.
Thanks
I have an old combi boiler (Worcester 240).
There was a timer next to it. And in the lounge an analogue thermostat with 2-wire connection.
I've changed the (lounge) thermostat to a digital thermostat/timer (Dayton Digistat 3) and all worked fine.
So I wanted to remove the old timer and...