Personally I have very little time for people like OP, you don't necessarily have to understand the technicalities of the work being carried out, just the the costs of a professional to carry it out correctly.
I believe they don't need to research the overheads and costs we incur each year to trade as heating engineers, common sense tells them that £400 less the material costs would not leave enough to cover our overheads, let alone provide a reasonable profit margin.
That is why they are on here asking questions and the other quotes are so much more, surely?
If you think that your research has lead you to understand what is required for your new boiler install then carry on, why rely on anyone else but be warned as Mr Ruskin said-
“When you pay too much, you lose a little money — that is all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing it was bought to do. The common law of business balance prohibits paying a little and getting a lot — it can’t be done. If you deal with the lowest bidder, it is well to add something for the risk you run, and if you do that you will have enough to pay for something better.” John Ruskin