I would say the boiler price is relevant to this thread, as the OP was asking about prices. I replied to you because I couldn't find a boiler that cheap in the Plumbfix ( Screwfix) book.
You also haven't mentioned any materials or sundries for this job. ( pipe, fittings, inhibitor, fixings, flux...
Fair play to you, but the price you said and your first comment on this thread hasn't really helped the OP one bit, this thread isn't about you getting a deal because the OP hasn't got a gas installer mate called Tony.
Not sure when you bought the boiler but today's price isn't £400.
Oh really??
I'm a plumber with over 25 years experience with my own business.
Any professional company would not be charging £700 for even a straight swop combi.
The boiler you have fitted is currently on sale in Screwfix at £474.33 with a free flue.
Did your gas installer do a power flush...
I think we need some more details.
Either you had a very good friend do this and they made no profit whatsoever.
Or someone working for a company did this as a cash private job and ( could have) used stolen materials.