To flush the system turn your rads off and the water supply to the expansion tank, go to the low point and you will find a valve that you can put a hose on to. Drain off some water more than what is in your expansion tank.
Buy some radiator cleaning soloution from B&Q, use the sentinal stuff. Put it in to the expansion tank and try and pour it through the hole so it goes straight into the pipes. Leave this for about 4 to 6 weeks and it will take all the rubbish off your pipes and rads. I put 2 cans in mine and left it for 3 to 4 months and it cleaned it out well.
Don't forget to turn your supply back on and then open your rads, turn your pump up to max so the soloution goes well around the system.
You may be better waiting with this in until may or June when it gets warmer.
Then do the same again, turn the rads off, both ends and empty the water from your system and open the rads when you have the hose connected and it is draining through the hose. Mine was a pain as they put 3 valves in and I had to have all three hose points connected.
When the water is draining turn the water back on and then let the system flush itself through.
You may want to take the rads off when flushing so you can put a hose on them to thoroughly wash out. When you put them back on put some PTFE tape on and wrap it in the oposite direction to thread.
When you go to fill the system back up, turn all the rads back on and pour 2 tubs of Sentinal protect back into the system before adding water and this will help protect it from happing again quickly.
A good plumber should charge 200 to 300 to do this, the gas board will charge about 2000+ for something that is very simple.
Good luck
Charlie