If the 'flow-switch' isn't making, it DOES NOT REQUIRE REPLACING and the price is extortion, if it fails the rubber diaphram sometimes needs replacing (about 5 euro) and
block of the valve may require some cleaning.
Suggesting the pump is the issue is frankly rubbish, if your circulation is fine, the pump is fine, the normal issues that cause this problem is either a blocked return filter, a blockage in the pipe leading from the heatexchanger down the return pipe to where the thermister sensor is located (will need to be removed and cleaned also.
As you say limescale, and your heatexchanger was changed 2 years ago, if your system wasn't cleansed and flushed at the same time, my money would be on that your new heatexchanger is now blocked too, replacing the old one on it's own was like a band-aid, as the new copper heatexchanger would act like a magnet for all the limescale in the system if not cleansed and would not last half the time of the original
Lots going on here, ring me during the week and I can discuss your options with you, see if we can put you on the right track