Hot Water but no CH Potterton Netaheat Profile 30e-80e?

In the last day or so we have noticed that we are getting hot water but no Central Heating (rads are cold etc.). After a brief examination we've noticed that the Boiler is being 'called to heat' and then before rads get warm the bolier shuts down/goes to standby. The Pilot Light is working Okay. The Fan was replaced approx. 3 years ago, a new Drayton 'Room Stat' fitted approx 2 yrs ago and most recently (3 weeks or so ago) a new Drayton Timer Programmer was fitted (faulty Advance switch on original ACL model) and the system was working well through October. The system is 18 years old so I guess it's to be expected that things will wear out from time to time. I have read through the various Q's here at How to Mend it.Com and their associated threads and none quite match up to this current fault (i.e. comments re 'faulty Mercury Vapour Valve', 'faulty R14 Resistor', 'faulty Overheat stat', 'Empty Header Tank', 'Pump' etc.). The 3 way valve was checked, with pump 3 weeks ago when diagnosing Prog/Timer issue and all seemed okay. The Room Stat is clicking with call to heat and so is the Bolier Thermostat (Thermostat Control Knob on Boiler) seems also to make a audible 'click'. The Boiler is hot and heats water okay. We will check Header Tank (checked every year anyway). Could it be the Overheat Stat. as it seems - at a glance - that the stats are overiding each other? I supose that there is an outside chance that the Programmer/Timer maybe faulty but it is brand new and was working fine. If fault is diagnosed before answer is received here I will post our findings. All help gratefully received, many thanks in anticipation.

Yours...cold in Swindon!