In message , Tony
Lennard writes
I am trying to replace my central heating timer. It is an old model I
guess from the 1960s, it branded Potterton, but inside it says
manufactured by Hortmann, model Q mk 3.

Does anyone know what the connections are? Horstamnn could not help.
Otherwise, I am upgrading to a Horstmann C27 Programmer, is there an
easy way to know what wire to connect where? - I have a puped central
heating sytem, with gravity hot water. There is no cylander
thermostat, so I have 2 wires to the boiler, and one to the room
thermostat.

Put a photo up somewhere, I might be able to identify it

--
geoff