Andrew pretended :
Are they fitting a magnetic filter to keep crud out of the new
boiler ?. They should and might be relying on this to collect the
rust particles.

Noiler manufacturers seem to insist on one, for the warranty. They do
only collect the rust particles - no use at all for collecting other
debris, which can still obstruct a modern boilers water passages. So
basically - if it needs flushing, it needs to be flushed before a new
boiler is fitted.