A further thought, in addition to my previous answer below, you could buy a new digital controller that fixes to the wall so you have more programming options. The wiring plugs into the same connector that the old mechanical timer connects to, so it is a really easy 'plug and play' task.
Rupert
May 2020
I'd suggest simply replacing the timer unit, it's an easy job. If your timer is the version with the little clock hands in the middle, buy the updated "MT10" version as a replacement. They fit the same way but are more reliable. Part will cost about £60
Rupert
May 2020
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