Ok guys.
I can't believe so many people are willing to think this thru and offer advice!!
Just to be clear; all rads have new valves, some are TRVs and others wheelhead.
I have fitted a brand new pump.
I have drained down twice in 48 hours and the water coming out of the system is lovely and clean.
Before fitting the new rad I fixed a hose from the o/side tap ( off the mains ) to both valves + blasted thru to try and clear air or blockages.
The pipework to + from the rad is in the 1st floor void between ceiling + floor.
It is a long horizontal run I'm sure + it could be an air lock. It doesn't "feel" like air though, because it doesn't splutter when I drain some water from the return pipe + air doesn't collect in the rad, which I would expect if it was being shifted by the hot coming thru.
It could be a blockage but I'm puzzled as to how the hot water can get thru + continue to work for 2/3 days.
I have taken a board up to reveal the pipework + turned the heating on, in the hope of feeling hot water in the flow pipe and establishing where, exactly, there may be an air lock or blockage.
However, I can only access about 4 feet of pipe, then it goes over a
wall. The pipe is cold for as far as I can feel.
I'm convinced that the problem is
a) a long pipe run including having to go over the wall,
b) a tee joint that the hot water is by-passing ( but which I can't get access to without taking up carpets + floorboards + which I'm trying to avoid.
c) a lack of circulation caused by a + b
Now my plan is to drain the system and fit an automatic air vent in the horizontal pipework, hopefully solving the problem of air in the pipe.
At the same time, I was going to fit 2 full-bore iso valves in the vertical section of both flow + return pipes so I can later fit a towel rad with a heating element in, without having to drain the whole system and losing all the inhibitor.
I also thought it might be a cheap alternative to fit 2 tees in the vertical return pipe, connect a drill-driven pump between them and use it every 2/3 days to assist the circulation in the system.
I know that seems extreme but it might be enough to get us thru winter and the rad may work more continuously when the weather turns cold and we have the heating on longer.
Thanks again everyone and the more I discuss this, the more common this fault seems to be. It's nice to know we're not alone!