Vaillant VCWGB242EH GC 4704413 Combi goes out immedi?

Hi, am working on the above boiler, was leaking at the auto air vent so tested the expansion vessel to find it perforated, fitted a new one into the system to prevent me taking it of the wall on the return side, now hot water works fine as before but the central heating will only work when it feels like it maybe every other day or it will ignite when I shut the hot tap off and ignight for about one secong, any ideas it has me puzzled Thanks.