How can I Fix My Ravenheat 820/20 boiler?

My ravenheat 820/20 boiler is supplying hot water but no CH. After a basic fault finding mission I discovered that the system top up valve had been knocked open. The boilers symptoms where that it was running a pressure of 3 bar but when swithched on the pressure rises to 4 thus tripping out the boiler incuding the pilot light. Im wondering if the expansion vessel has been damaged due to the excess pressure. I tried draining the system down to zero in order to top up to 1 -1.5 bar but pressure would not go below 2.8bar. this is when I discovered the top up valve open. Managed to drain the system, brought the pressure back up to 1.3 bar. Flahed up the boiler and it started making a knocking sound, the pressure rose quickly to 2 bar and it tripped again. Left it overnight and started it again this morning. Made knocking noises, Tripped and pressure relief valve blew draining system to 0.5 bar. Any help would be grateful