I'm sure you'll all shoot me down but I think £840 for a days work is obscene, even if the fitter is paid £200 theres another £640 to cover guaruntees etc. I fit ventilation and the labour charges are a million miles away from fitting a boiler...
Just because your boiler was badly fitted in one day, why do you think that it takes only one day to do the job properly?
Have you heard of power flushing? That takes 4-6 hours and uses about £40 worth of chemicals.
Its because your illegal installer did it all quickly on the cheap and did not clean the system that you now have all these problems.
Tony