dear gasguru, sorry about the time to reply, had to work on the car, new track rods ends and balljoints to pass the MOT.
I have now checked the switches as you suggest.
When DHW is demanded the valve operates and the contact screws come out clear of both switches. I have run it through...
I have a VCW GB 242 EH boiler, the hot water works fine, but the heating is intermitent.
Sometimes it works normally, other times it does not fire up when switched on, sometimes the heating comes on after running the hot water for a short period...............about a kitchen sink full...
I was thinking of one of these also, unfortunately if you read the instructions you will find a set of gas cylinders only last about 20 minutes.
and at about £8 each per refill it is not cheap!!!!!!!!!!!
I went for mig in the end and am well pleased with it.