Right, here goes. The boiler is a Worcester Delglo. It is situated in the airing cupboard and in effect, the burner is situated directly under the hot water tank. There is a header tank in the loft and is at its normal level. The pump, next to the boiler, is working.
As I mentioned, there was no problem till I turned the heating on for the first time. I sought advice from a couple of friends who know a bit about plumbing. One suggested the trv's were suck, the other suggested an airlock. I have removed the 'knob' of the trv's and pushed in the pin in an attempt to release any stuck pin. The only exception to this is the larger rad in the kitchen as I cannot get that one off. I then turned all inlets off with the exception of this lager rad which is the furthest one from the boiler. This resulted in the two rads in the living room getting hot!! The heating is on at the minute. Some rads are hot, others ae warm but the pipe on the inlet side of he valve is hot. The two rads without trv's, bathroom and ensuite are warm, I can keep my hand on it all the time.
The boiler has been in the house since it was built, about 25 yrs I think.
I will probably get it changed but had intended to wait till after Xmas.