There's no easy answer to that question, but if all other conditions are right, you can get away with a fair bit of debris and sludge left in the system.
One of those conditions is that the pump is in the correct position relative to vent and cold feed, and I'm a bit concerned at your description because it implies that this could be wrong on your system
I am fairly confident about pump position - sequence is boiler , vent , cold feed then pump
If you've had corrosion then it's been for a reason, which is commonly either:
1. System needed inhibitor but didn't have any in;
and/or
2. Inhibitor was added but air was being dragged in, which negated its effect.
Inhibitor was added by a plumber - but how much unknown - this was as a result of air in the system.
As long as you have flow, and as long as there is no pumping over, then I'd drain the system to clear out what you can, and refill with a dose of inhibitor.
All radiators at full temp - no radiator issues. I have read in a Myson Aerjec product sheet that if the feed pipe does not have a large enough capacity ie 2% then air can get into the system. My feed pipe is 15mm and 5 foot to F/E tank. System comprises 15 radiators - installation 10 years old
In the spring you could start the process of removing each radiator and flushing it out with a hose in the garden. Or, you spend a small fortune on powerflushing. But that carries the risk of exposing latent leaks.
In the meantime, if you do drain down, while the system is empty you could fit a Magnaclean and be gradually pulling more debris out of the system.