Potterton Suprima 80

Check that the primary water is actually able to flow well - stuck pump?

No, the pump is setup for fast speed, if its med or low the heating just gets warm. Only on fast pump does the rads warm up. There is no air in the system as the plumber has removed it all. Still same problem Temp control knob on boiler is set to max and the boiler does not stay on enough to heat the water out from boiler above 60degree.

I'm sure others have seen this fault before. The temperature sensor(s) may be out of specification. I'd go for exchanging one of those first you could also compare the replacement unit to the installed unit to see if that is the problem.

is there any chance that the temp control knob device is faulty?. I know i had replaced the circuit board, but the temp control knob itself is that on the circuit board or is it a seperate device I think its called a potentiometer!

It's on the back of the control panel, the knob's on the front. Just next to the reset switch which you should be very familiar with if you're the proud owner of a Suprima. I think that Suprima temp sensors are very prone to cracking