original guvnor said:... mine was a bit more complex because it also involved relocating the boiler from the kitchen to the airing cupboard with the all hassle of re-routing the gas supply and relocating all the controls too.
I think a relocation of the boiler is bound to increase the work required as you've found out with the revised Building Regulations now in place, so an extra £1000 should not be unexpected. However, you clearly did the right thing by getting a few comparable quotes.