I have a well-known firm installing a WB boiler today, to replace our
old boiler.

They say that their normal way of doing things is to connect up the new
boiler, add chemicals, and flush the system for an hour. Then drain and
refill.

This seems all wrong to me, and indeed the WB installation manual says:
"A new appliance must only be used to assist the cleaning of a newly
installed system."

Surely, they should have flushed the system with the old boiler, or do a
power flush?

Any suggestions about what to do?