My Vaillant E 242 E will not egnite?

I have had a house in Spain for 6 years which has a Vaillant E 242E installed for hot water and central heating. It has generally perfeomed well but lately has started playing up. Firstly the heater gets used very little. Mostly only being asked to work for water and then only around 3-4 times a year for a week or two at a time.
The unit now will not egnite whent e water tap is turned on. Thsi has happended before and soemone I know made an adjustment which caused it to work, but the problem has returned. I can hear the unit start to click but no pilot light or flame. Also the manual ignition does not seem to work either to start the unit.
Any help or advice gratefuly received.