Ferroli Modena 102 Temperature Problems?

I have problems when trying to adjust the temperature of both the DHW and the CH. Both are very high and do not seem to respond to adjusting the dials on the front control panel. Also the boiler pump and burner continuously cuts in and out at the same time when on CH only. I'm sure the pump is supposed to run continuously with the burner cutting in as required to maintain the return temp. (Unless someone calls for hot water)

Service engineer previously came out and changed the venturi, removed the flue discharge restrictor, cleaned it up and changed the thermistors. He said the only other thing would be the pcb ?

I don't think it's a problem with any blocked flow restrictors etc. as we have soft water in our area.

New pcb's are very expensive as genuine parts and i would like some assurance, if possible, that this is the problem before paying more money?

Also I have found a website (CET Ltd.) which sells second hand pcb's for £50. If i get this does anyone know how easy they are to change ? Thanks in advance for any help !