Ravenheat Fanned Combi RSF 82ET - Hot water but no Central Heating?

Hi there, my ravenheat combi works fine when I turn on the hot water taps but the central heating has not worked for several months now.
However, yesterday I was cleaning the kitchen near the boiler and I guess I must have knocked something because the heating started to work, albeit briefly. The boiler was set to constant (on) and I wanted to change it to the timer, if the heating was about to work again, so I changed it to auto mode. As soon as I did this the heating ceased to work again. I then thought I would give the boiler another light knock and see if this would bring it back to life again. When I knocked it on the bottom left hand side the heating came back on. I then switched the boiler off at the mains and back on again. Same result - no heating. I have since been unable to get the heating to work again. The hot water is fine, apart from the odd occasion when the flame failure light will signal. To fix this I just have to switch the boiler off and on at the mains. I am thinking to get a plumber in to have a look but I thought I would just see if I could gauge a general opinion on what the problem might be first of all. Many thanks, Andrew.