How much you pay for a new boiler will very much depend on who is installing the boiler. Most people feel that as long as the gas plumber / heating engineer is corgi registered (now changed to gas safe ) then it will be OK, this is not the case. There is many cowboy plumbers and gas fitters out there that will miss bits off the quote to get the job.
Customers think that when they need a new boiler it’s just a case of a simple boiler swap. To the new regulations there is no such thing as a simple boiler swap any more. All new boiler installations need to be installed to the new energy efficiency rules which means boilers now have to be A-rated. A-rated boilers are more commonly known as condensing boilers.
Cleaning out the old system is vital with modern boilers as the heat exchangers tend to be so much smaller and can block up quickly. power flushing the central heating is the best way and I would also advise in fitting a Magnaclean filter.
Many of the new boilers will need an updated gas supply pipe (something the cowboys normally miss).
A qualified electricians should be used when installing a new boiler. ( The plumber sometimes wires this them self to save on costs not allowed )
Your installation may also need updating to a fully pumped system. When you have had your new boiler fitted you should get a building regulations compliance certificate. (you will need this to sell your house) So taking all this into account you should be looking at paying no less then around £1500 for the work to be done. (if it is less than this ask yourself why).