I was asked for a quote to fit a new boiler, add pipeworkt to fit a magnaclean, change a radiator like for like, add plastic pipe work for condense and fit a new kitchen tap.
I quoted £400 for labour only.......... fair or not fair?
If they're supplying the boiler you'll register it,then make sure they understand you'll guarantee your work, but any problems with their boiler & they'll have to sort it or pay! As you'll have already lost out on your uplift of profit on it.
It depends on where you are and how long it takes, I'd do a combi swap in a day and a half including a chemical flush and would consider £400 labour a fair wage. (North Yorkshire).
Its better to be busy... Classic supply and demand, people in cities with loads of customers and not many plumbers can charge more