I have a strange issue with my central heating. Moved into a new house
3 years ago and heating worked fine for a year, then last winter we
had an issue where it wouldn't turn off.

We have a standard Gravity Fed System Setup
Potterton Profile (in garage)
Electronic Timer (in kitchen)
Thermostat (in Hall)
Zone Valve which switches between water and Heating (in airing
cupboard by water tank).

This is an intermittant problem and sometimes it doesn't happen for
weeks then happens everyday for 4 or 5 days its very hit annd miss and
unpredictable.

So when the heating reaches the correct temperature or you turn the
thermostat down the heating stays on, even if you turn the timer off
(for the Heating and Water). It just keeps running and the house gets
hotter and hotter.
We have to resort to flicking the main swich off so everything is
shutdown which is annoying.

I've had several things done to try and fix the problem but none has
worked:

1. replaced the Frost Stat in the garage (also bought a thermometer
that shows high and low temperature to see if it goes below the 5
degrees, and it doesn't).

2. Plumber suggested changeing the Zone value, so we did and this made
no difference.

I've had a plumber out and they don't know.

I could obviously replace the Timer and thermostat but then i'd have
change every part of the system, except the boiler.

Has anyone seen this issue before and have any suggestions to fix,
other than just replacing the boiler?

Regards
Stuart