Vaillant VCW, no hot water or heating! help desperatly needed?

I have a vailllant VCW GB 282E and the hot water and the heating do not work. I havnt had the boiler on for the heating since about march and the hot water has been running fine untill recently. The burner doesnt ignite but once in a blue moon it does but will only last for around 3 minutes. The hot water doesnt work at all at the minute. The servo valve is moving when the hot water tap is turned on and the micro switch turns everything on but no spark. i have swapped the whole electric system with another from a known working combi and the fault is still the same! i have tested the ignition unit and it works fine. I really could do with a hot shower! i called out an engineer and he looked at it and was stumped! it has to be something small for everything else to be working fine! even a manual might help. tried through vaillant website, what a stupid system!