I've recently had a number of quotes for the installation of a new
condensing combi boiler and new radiators. Currently, we have an old
gravity fed system with 20 year old rads.

Some of the plumbers we've spoken to strongly suggest chemical or power
flushing the system and fitting a filter on the return to the combi.
However, the plumber we are feeling inclined to go with suggests just
letting the boiler run and water flushing. He said that he hadn't heard
of people fitting filters on the return.

Is it necessary to have a filter on the return if the system has been
water flushed and the radiators are new?