Central heating blues

[Radar]

Hi
As the winter approaches my Potterton primaB boiler has decided to play up.
The problem is that I can have hot radiators and luke warm tap water, the temp on the tank 65 and I have changed the temp stat and still the same problem. Also I cannot have the heating on separate I must have hot water and heating to be timed together for the radiators to work. I was wondering if it the valve at the base of the tank is faulty- when I turn off the main power supply switch this valve whirrs and the lever retracts (is this normal) so the valve is not siezed, power up and the lever returns back, I thought that this may move on what demands I need hw or ch but it only says in one position (its a landis &Staefa unit).
If this is the fault can you buy overhaul kits or separate parts (as just the electric side as the valve is working and makes life easier buy not draining the system to renew the complete valve or is the renewel of the hole assembly the most guarenteed cost effective route?? The system is only 4 years old so hoping like for like parts are available ??

Anybody help

Thanks

RADAR

 

[MORGAN69]

CHECK HEADER TANK IN LOFT,ALSO COLD FEED INTO SYSTEM,MAY BE BLOCKED.