Vaillant VCW GB 242 EH?

I have a vaillant vcw gb 242 eh boiler that was already in my flat when i bought it, i have no idea how old it is but it worked fine. then last november it started trying to ignite over and over again for both hot water and central heating, then cut out as it couldnt ignite. Got a corgi repair man out and he took the boiler apart, put it back together and said he cleaned an electrode as the boiler was cutting off when it got too hot as the flame sensor electrode was dirty and the boiler was not igniting when it got too hot. Charged me and left, the hot water had been turned right down and the water temperature was lower. The heating worked fine but the water was only luke warm,.

turned up the water and it was ok for a bit then last month the water went luke warm again, couldnt turn the water temp up any more so got an engineer out, the gas pressure was low so he charged me £50 to adjust the gas pressure. Then he went, seemed ok but about 12 hours later the ignition started clicking but the gas wouldnt light. It was going off then relighting quiclkly. the water was going hot and cold. Central heating was fine. he came back and adjusted it again, then the water was fine but the central heating kept going off and triping the safety switch on the front of the boiler. came back and changed the thermistor and adjusted it again, worked for two days now its started the old problem of trying to ignite over and over and then tripping the switch on the front. It is ok on low heat central heating and the waters is ok for a while if the heating hasnt been on but otherwise just switches itself off. I had paid another £90 for the final 2 visits but its not fixed. i have found a few posts on other forums citing the same problem with this boiler but everyone says they will only post an answer if emailed a corgi number. I dont want to fix it myself, just a vague idea of what might be wrong as i am fed up of paying out call out charges for the problem not to be fixed. Please can anyone point me in the right direction of a possible cause of this type of fault?