Halstead Best 50 boiler problem

[Mahadev]

Hi Guys
My boiler has developed a fault which is puzzling me.
As from yesterday the boiler will ignite normally and run for 3 to 4 minutes and then close down. I can hear the pump still running and the HW thermostat is demanding 'heat'. All the radiators remain cold, the Thermostat is still demanding 'heat'.
The problem occurs both for HW/CH or just HW.
I might of suspected a problem with the 3 port valve, but the problem still exists if I manually open the valve using the small 'lever' on the valve head.
Any ideas would be appreciated.
Thanks for taking time to read about this problem.
Mahadev

 

[Mahadev]

Dear All
Quick update. I noticed when manually opening the valve the lever had not locked into the open position and had presumably closed. I reopened it and found the boiler ran normally.
So I guess it is the 3 port valve not working. See my post '3 Port Valve'.
It appears to be a fixed head, so what can I do to fix it?
To remove from the system I will have to cut 22mm pipework because of the way it has been mounted. Duh!

Has anyone successfully fixed a valve in place?
I believe one can get hold of a new motor and micro switches, but from where?
Thanks for your time
Mahadev