"geoff" wrote in message
...

"The problem is that it's very old and the plastic knobs just get
pushed around the dial so the heating wither stays on, or stays off
depending on which knob actually managed to switch it last."

gives it away as being a 102 or a 103. The direct backplate compatible
replacement is the 103E or the 105E

To replace it with any other make would involve replacing the backplate
and rewiring it

We had an old Randall with the same symptoms, and bought a Drayton Tempus
Seven from B&Q, for about 40 quid.
It comes with a replacement backplate and a simple wiring diagram, and was
pretty easy to fit. It works a treat.