vokera 20-80rs flowmatic no heating?

hiya, up untill yesterday ive never had a problem with my heating all of a sudden when the timer finaly gets to the time for my heating to come on it does not fire up, i tryed setting it to constant still no diffrent. my hot water is working fine and if i turn my heating onto constant and run the hot tap then turn it off again the heating will stay on for 5 mins but then turns off again?

Any ideas on what the problem could be?

p.s just had new motor fitted at the beginning of the year.