Digistat RF2?

I have digistat RF2. It is set to come on at 20degrees at 5am and go off at 9am at 8 degrees and come on at 3pm at 23degrees and go off at 2300 at 10degrees. The problem is all seams well, but the grant boiler does not come on. I have checked the pressure gauge and that is fine. I have checked all pipes and fine. I currently have to manually swith on via the fuse box and it will come on for a while but then switches off. The signal and batteries are fine. I spend 50 times min an evening walking to the swith box and flicking switch which makes it come on (both lights on boiler) but then top light goes off within 3 seconds but bottom one stays on hence no hot water or heating. Help Help Help. I have had an engineer 3 times who thinks its fixed but come the evening it all starts again by just not coming on and when I get it on goes staright off. Please help