F wrote:
I need to replace a (second) radiator after a pinhole appeared in it. It's
one of the original 1970 radiators, others being added in 1983 to make a
total of 16. The system was flushed and had inhibitor added quite some time
ago now and I'm sure it's due for another clear out and another dose of
inhibitor!

The absolute best way is to remove the radiators, and flush them outside
using a hosepipe.
Then add a filter, and flush the system conventionally.