Pottertton PUMA 100 - Heating timer?

I have a bolier that used to work ok with the Bolier set to standby and the Heating switch set to timer.
Now the heating doesn't turn on with the timer and I have to put the boiler to 'ON' to get the heating working. The heating switch can be left on Timer.
The timer looks to be turning ok.
If it helps with the bolier on 'ON' and the heating on Timer, the heater will turn itself off.