a good boiler for little money is a potterton gold 28he, less than £800 inc clock flue and vat, its british made, parts are cheaper than others and ready available, easy to service, i have fitted 100's and if trouble occurs( it does with all boilers) heateam are the back up engineers, and its guarrenteed for 2 years, it also had good write ups by which magazine,
Its not just 2 days work, joe 90 there's things you don't see, like going for the boiler and materials, visiting to quote, revisiting in case of things going wrong, ( the plumber is the first to get called, no matter what) , paper work before and after the job,
a fair installation price is £700 ish, depending on the gas pipe needed in my opinion,