Thanks for the advice.
It's strange that the system has been installed for about 20 years but the problem has only developed in the last year. I'll have to keep on looking.
When I run my central heating system on "Hot water" only the radiators at the furthest part of the run are getting hot. This is a problem during the summer as I have to switch off the Central heating system and run on the immersion heater. My system is open evented, fully pumped with seperate...