Erratic heating

Myson Orion Boiler, Potterton ep2002 timer.

Boiler does not always fire up when timer activates it. If we mess about cycling the clock and changing the settings on the timer it usually fires eventually. Timer has been changed and thermostat clicks when turned. Boiler about 15 years old. Where do I go from here?

Many thanks

Get someone in that understands the fact that it is not the timer or programmer that starts the boiler (in a pumped system). That would be a start.

Make sure you pay on either (1) a successful diagnosis of the problem or (2) a repaired system, assuming you agree to the price