Thanks to the advice here and a particularly useful pm from Mr andyoxon of this parish, I reckon I might have sorted my errant CH system. The problem was the boiler 'Kettling' probably due to low water circulation and crud build-up.
So, eventually drained the system and added Fernox cleaning gunk, refilled system. Still no water cirulation even after lots of bleeding and no heat dissipating form the boiler, Damn.
Part drained the system to get at diverter valve and pump. Diverter valve seemed OK. Had titanic "Man vs. inanimate object" battle to get the pump out though
Eventually shifted it. Throat of the pump at both ends fairly clogged restricting flow. Spent an hour or so chipping away at the black gunk and generally de-clagging the pump. Reassesmbled everything. Bled everything. Damn, some slight warmth in one or 2 radiators but still not functioning anything like properly and the boiler tripped-out on over heat. Great.
So I was all for calling a plumber on Monday morning... and then I decided to try it one more time...
Manually jacked-open the diverter valve to be 100% sure where the flow was going and a few radiators began to get warm, I could hear a lot of air in the system, kept venting, kept bleeding every rad one after the other, made sure every valve fully open... eventually only 2 cold rads left... YAY!
Copious bleeding later, all warm. Checked the hot water side and that worked OK too! If it all comes-on in the morning were pretty much sorted ( well, apart from having to flush-out the Fernox cleaner and fill with inhibitor).
So, clagged system improved by Fernox circulation, pump flow sorted by digging out crud restricting the inlet and outlet flow and an awfull lot of air expelled as well.
Lot of frustration, lot of time, got there eventually largely due to your comments.
Had a nice 2 hour bike-ride tonight to unwind.
Cheers all.
