I did a search on boiler prices. Saw that I could buy the boiler at 610 GBP including VAT. Meaning that with the labour and "all" materials (but not including electricals) the cost estimate is around 1900 GBP. I estimate that it takes half a day 4 hours to remove the old one and fit the new one. Say 5 hours on site work. Equates to 1500 GPB (taking out 400 for materials) for 5 hours = 300 GBP per hour.
sounds to me like you'll end up with bill's mate from the pub who'll turn up in some rusty untaxed sherpa and who'll be going through your drawers the moment your back is turned. you clearly havnt the first idea what you are one about. i suggest you get your head out of your arse.