I found a post from PigletJohn describing a DIY fix for sludge-filled radiators. It was posted in 2011 so I wondered if the advice had changed, e.g. if there were any new chemicals or filters on the market that were better than those mentioned (below).
And here it is...
PigletJohn Mon 14-Nov-11 16:27:05
What you are describing is caused by sludge. It is usually thick black sediment of iron oxide caused by the corrosion of the steel radiators. It is very common on old systems, especially if they are open vented and/or have not been treated with corrosion-inhibiting chemicals.
BG will tell you that you need a Powerflush, which will cost some hundreds of pounds. They will tell you that they are not responsible for your system being old and dirty, or for it not having been treated with chemicals. They will tell you that if you don't have it done, they will accept no responsibility for future sludge-related problems, and they will not be covered by any service contract. Servicing a boiler also does not include cleaning radiators. Sludge can block boilers, especially efficient modern ones.
An independent heating engineer will usually do a powerflush cheaper than British Gas, but it is essential to get a well-recommended one. In a typical house the job will take about half a day and involves a large machine with a powerful pump, and some aggressive cleaning chemicals. If you have a service contract, BG wil usually give a lifetime guarantee on their powerflushes, provided any piping faults are correct at the same time and you keep the corrosion inhibitor topped up.
If you or your DW are fond of DIY and can do basic plumbing, then you can do a pretty good job yourselves. You can buy a litre of Sentinel X400 for about £15 and give it four weeks to soften and break up the sediment, then drain it out with whatever has been loosened, then (preferably) take off all the radiators and hose them through in the garden. X400 is very suitable for DIY use as it is not acidic or aggressive and will not cause any damage. If in addition you can fit a Magnaclean or similar, it has a powerful magnet inside that will attract and trap any circulating black particles and prevent them causing future blockages. Magnacleans are spectacularly fast and effective, but BG no longer fit them as they tend to leak after a while. They have a 2 year guarantee and by that time they should have captured all loose aprticles in your system, so you could just have it removed then. Other devices are available but none are as spectacular in use.
On final fill after cleaning, be sure to add a litre of X100 or similar to prevent future corrosion.
Note that corrosion inhibitor chemicals are NOT cleaners, although the same companies usually make both, so adding an inhibitor will not clean out existing sludge.