I had some work done on my heating system - to fit a new radiator as part
of an extension.
Unfortunately I was out when the plumber finished his work and I cannot be
sure he added corrosion protection.
Ideally I would like to clean the system as I have one sluggish radiator
and I suspect the manifold may be a bit sludged.
Is there a cleaner that I do not have to neutralise?

Ideally I would like to add something that will soften any deposits and act
as an inhibitor.

Happy to drain off a bit of water from each radiator to get rid of debris
but don't really want to embark on major flushing and filling with the
added cost of cleaner - neutraliser - inhibitor.

System 28 years old (always dosed) Boiler 3 years old.