far too much involved in this simple job if you dont know how to dismantle it, ok you need to do it to learn but you need to do this type of thing with someone who knows what they are doing till you suss it out, as per usual the stock phrase is use the MI's, you will need both the boiler and fire MI's to get all the checks, in its simplist form it is an easy job, first thing to check is the ventilation BEFORE you even touch the appliances, any less than 90% of required vent makes it AR so thats your starter for 10, if vent ok, and depending on who you are doing the job for (Landlord etc, do a TT if required/desired/agreed etc etc etc) remove fire, clean around boiler and check flue connection, visual on catchment area to ensure all is sealed, gas rate boiler to see how it is performing, clean lint arrestor at injector, depending on gas rate strip clean inspect burner and injector, check burner pressure against data badge, increase BP to max to do spillage test, refit fire, strip and clean as required after gas rating, do spillage test FROM POINT AS PER MI, NOWHERE ELSE, some Baxis have specific wee match spigots at the left hand side of the heat exchanger on the fire, do another spillage test with boiler on max and fire on max, if all is well, check for extractors, tumble driers ceiling fans etc, then re do spillage tests as required, return boiler BP to previous setting, clean up and bale out after getting tea and cash, i bet i have missed something, if so someone will gently remind me!!!!!!!!!!!!!!!!!!!!!!!!!!!!!