£ 2800 labour for 4 days work... that's 700 per day...nearly £ 100 per hour...(lunch break?)
There are hopefully some plumbers out there who will charge an hourly rate less than that; even in London.
Maybe find a Corgi person with the best hourly rate, pay them for how long it takes. If they are nervous about underquoting , they will no doubt inflate the figure to cover theat risk. But as long as they don't waste time or re-do work they did wrong, then paying by the hour might be cheaper, no? Especially on a complicated job. It seems that the words "replace boiler" sets off a 'kerrchink' sound with dollar signs spinning in their eyes. Shouldn't matter what they are doing; a reasonable hourly rate is a reasonable hourly rate. £ 100 per hour seems like a great figure if you are at the receiving end, but should anyone really get paid enough to buy a new car every week?
Uh oh, here goes the political / marxist / socialist / Corgi overheads thread...
So you know the spec of the boiler, controls,chemicals and pipework that will be required!!!!
I doubt it because if you can get a vaillant 831,flue,etc for £700 inc VAT then you had better share this with us all.