It sounds a perfectly reasonable suggestion. Rust on the outside could imply corrosion on the inside. If I were you I would get an independent plumber to come around and give you an assessment of the heating situation it does sound a little to me like there is more muck in the system than perhaps you are aware of. Very early on in this thread somebody did suggest sealing the system once it is clean as long as your boiler is suitable for this
It just so happens I've got a company coming in on Friday to quote me for a sealed system, my boiler is not suitable for a sealed system and is quit old now, I figured if I had to pay BG 700 for a flush I may as well go the whole way and get the boiler changed and a sealed system, should come with a powerflush as well.