I am not qualified or experienced to be able to give you any help but that does not stop me trying...
If the boiler fires up then the boiler's OK.
In many installations the signal to fire up for CH only comes from the external mid position valve on an Orange wire. If the valve is defective or the micro switch that sends out the fire up signal is broken then the boiler might fail to fire.
Explanation here...
http://www.diyfaq.org.uk/plumbing/controls/midpositionvalve.htm
It says...
In the "heating only" state, both white and grey are energised (hence the need for a changeover tank stat, and a "hot water not required" output from the programmer). Regardless of the position of either switch, AC will be supplied to the motor, and it will wind to position B. In addition, SW2 will connect white to orange, switching on the boiler.
Peccavi, June 2011
