Intermittent boilet fails to fire on heating?

I have a 5 year old Glow-worm CX35i controlled by Drayton Digistat SCR and analogue timer (on the panel). The boiler is failing to fire on heating demand by the timer and also the 'on/off' switch on the analogue timer. It fires fine on hot water demand, so I guess the igniter is fine and the problem is with the SCR or the timer. I recently moved the SCR from one side of the boiler to the other (slightly further from the boiler casing but more in line of sight from the Digistat. Any ideas?