Follow-up
At the weekend I went to a local Co., where I live.. CORGI etc.

Very grown up chat about boiler sizing, whether to use existing
expansion thingy or buy a system boiler with it in-built and so on.

"Let's start with the fact that you know what the boilers cost", said
the bloke.
"We really do think power flushing is a good idea and since you've got
a biggish house with lots of rads the install will almost certainly go
into the second day", he added.

"So", he said, "I'm afraid we'll have to charge £4-500 for the labour
and bits on top of discounted price for the boiler".

This works better than £1,000 less than BG. These guys are CORGI
etc.,so warranties should be OK. When I asked when they could do it,
he said, (believe it or not!), "Well not next week, 'cos we're quite
busy but certainly by mid-end Oct!"

I've been treated like a grown up and I think I'm happy to give them
the business and since it' all been up-front, prepared to lob in a few
more quid, if necessary for unforeseens!

A bit of a no brainer really. He didn't try to sell the "British Gas
own brand" being the only boiler offered by the BG salesperson but
discussed their experience across the range.

Hooray for the liitle bloke!