The dirt that came out from my radiator!

pieroabcd

Sorry, 8 was away.
There's really nothing that looks like a filter and the lower panel is fixed.

WIAWSNB

In that case I'd add strong cleaner like X800, and run the system when the weather demands it for a couple of weeks, and then drain fully down - all rad valves open, and bleed screws too - refill, run, and repeat. Keep an eye on the water colour...
Or, if you could bear it, run the cleaner for a couple of weeks, drain down fully, and remove each rad for hosing-through outside.
When you replace the boiler, a filter will need to be fitted, so personally I'd try and find a way of doing so now - start getting the benefit.

pieroabcd

Isn't it better done in spring? Now I would have to drain the system just days after this cleanup.

As I suspected, replacing the lockshield valve did a lot of good. That radiator now becomes totally hot much earlier, clear sign that the previous lockshield didn't let enough water out.
It looked too basic to be any good.

WIAWSNB

You can do it any time you like. Using chemicals requires the CH to be running so it can circulate hot water + the cleaners, and you don't really want to do that in balmy weather.