If the water is moving but the radiators is not all the way hot, it sounds like you need a power flush asap. Problem builds up as the sludge forms and gets more and more.
I agree that power flushing is a very skilled job where you needs loads of training and experience. Have done over 250 myself and I am still learning new things. You can't just watch a youtube video and think your an expert, you will more likely cause damage before you do the power flush properly.
It's not good. Make sure the filling loop is shut, then bleed a radiator to reduce the pressure it will take a while to get it to between 1-1.5bar. If the pressure goes back up you need a professional to have a look at it.
Yes it can be power flushed, but it's recommended by the powerflush association that you do not use acid to do the flush, that the company do NOT connect to the boiler's internal pump as it can cause damage and that if you are having hot water problems. The plate heat exchanger should be...