ha ha, yes it is
if the small "feed and expansion" tank has brown mud in the bottom, that is sediment of fine rust particles caused by the water inside your steel radiators. So is black water and sludge in the radiators.
Things you can do to reduce the sludge, scale and sediment:
Have a filter (usually magnetic) fitted in the large return pipe to your boiler. It will gradually trap circulating particles from the water so they can't collect together and form a blockage. The trapped sediment can be emptied out periodically. It is no harder than emptying a vacuum cleaner, once you know how.
Add an corrosion inhibitor to the circulating water (this scavenges spare oxygen from the water which accelerates the corrosion). Over time it gets used up. If fresh water is added to the system, especially due to a fault or blockage causing pumping-over, but also to replenish minor leaking, it gets used up sooner.
Assuming you have not done these things, it will be dirty and sludgy.
If you are having your boiler person round, ask what they would charge to fit a filter. They typically cost around £100 plus fitting. It is usual to drain the system down before cutting the pipes. Preferably a sludge-loosening chemical is circulated beforehand, so that draining will wash away some of the old sludge.
if you are fond of simple plumbing jobs, you can give it a chemical clean yourself. This is best done before a blockage has occurred. It will cost a couple of half-days of your time, plus around £32 for chemicals. In a hard-water area, an extra £32 for the limescale cleaner. If not, you can pay the boilerperson to do a powerflush. This will be quicker and more intense, and will cost hundreds of pounds.
I'd suggest starting with the filter, because it will stop things getting worse. Magnaclean is very effective on a dirty old system, and the newer ones seem to have got over the earlier leaks trouble.
more modern, sealed systems (with no feed and expansion tank) are less prone to sludge unless you have a leak necessitating frequent top-ups.