Why won't my central heating fire up?

I have a Worcester 28CDI which works no problem for the hot water. However the central heating won't fire up. If I tweek the central heating adjustment on the rear of the PCB then it fires for 30 secs to a minute before cutting out. Tweek again and it works 30 secs to a minute before cutting out and so on and so on. I would never get out of the cupboard if I wanted to heat the house. Any idea's, can anyone help, please?