Heating system not working correctly?

My central heating system is causing me problems. The boiler fires OK, but I can only get hot water. The central heating comes on for a few seconds and then the boiler switches off if I program it just for CH.

The pump seems to be working (it’s a Celsia Multi Head). The programmer is a Lifestyle LP112 and is not that old.

I took the circuit board out of the 3 way motorised valve (I was convinced this was where the problem was), but nothing looked obviously wrong. The micro switches seem to be OK. The valve is an ACL-Drayton 679H340-30LO

Is there any way I can test the valve/micros switches for correct operation - preferably without unsoldering them from the circuit board! The spindle in the valve seems to move OK.

I have drained the system twice. Frustratingly I managed to get it going last Sunday, but since then it has resolutely failed to work properly.

Any suggestions would be gratefully received.