Thanks to everyone for your advice.
The problem was the valve sticking and only partially opening up to the central heating circuit. I removed the motor and exercised the stem a few times and it loosened up nicely, this gave me almost instant hot radiators throughout the house. I have tried...
Thanks for the advice....
I have tried the pump on all of the different speeds and it doesn't seem to make any difference.
I have also tried to balance the system (without thermometers, so it was a bit of a guess) but as I started at the radiators nearest the pump i.e. the upstairs ones...
This is a simailr problem to others already posted so my apologies in advance for any duplication.
I seem to have two problems:
1. Downstairs Radiators not getting hot
2. Central Heating switching off when Hot Water turns off
I am trying to understand how the system is supposed to work...