I know from your question you are no expert. No offence. Is it open vented or a sealed system - I suspect open
before spending your dosh - check if the water flow rate through the boiler is high enough AT ALL TIMES -- make sure the pump is on high speed - and or a bypass if you have one is sufficiently open. Forget bleeding the heat exchanger - it will be full - -leave heat exchanger alone. You must check flow rate first. Then you can try your sentinel chemicals. Don't forget it is possible to have faulty heat exchangers from new. Believe me I worked for the one of the biggest manufacturers for 40 years.
from Dave 45 years experience -if you lived near clitheroe I would call
dave broadbent
October 2006