Safety Ground (Earth)
Switched Live from the Timer for heating
Feed to fire the boiler for heating

Replace with a digital thermostat. Much more accurate than analogue types.
Almost all digital thermostats only require the timer live (in) and demand
for heat (out) connections. Earth, neutral and permanent live are not
usually required.

The earth may be a neutral for an old analogue thermostat, naughtily using
the green/yellow or bare conductor. In any case, the digital thermostat
won't need it, so it can be ignored, provided it is tucked away safely
insulated.

Christian.