Issue with oil fired central heating

[CarlMc]

Hi. Currently experiencing an issue with our oil fired central heating system. When CH/HW is switched on, the central heating does not always work - the hot water does however without fail. We had the timer set to come on last night but alas, only the H/W came on. After a couple of hours of playing with the controls - we could not get the central heating to work at all. This morning however, both worked - quite strange! Furthermore, sometimes when the C/H does work, only half of the radiators in the house heat up - when this happens, we turn the CH / HW off and then back on again immediately and hey presto - those radiators then start to work. Wondering what might be causing this issue. The controls are quite old (Honeywell st6200a) so might need to be replaced, but I'm convinced that there's something else that's not quite right... any advice would be much appreciated