About 8 months ago, I began to suspect that the system was silting up as the "new" radiator was no longer getting hot. There's a standard Honeywell 3 port diverter valve serving the hot water / heating demands and a room thermostat sets the temperature in the lounge. I checked that the diverter valve was OK, but there was still no change to the cold feel of the radiator. (The radiators which get hot with the hot water priority in action don't seem to follow any particular logic, and I suspect that it may depend on how the various lock-shield valves have been set.)
In the last few days, the radiator has got hot again and I haven't done any flushing out of sediment. (Was waiting for warmer weather...)
What seems to have happened is that some "logic" in the control system, an old Randall 922, has been correctly reset as a result of my briefly turning the mains off to the Randall 922. When it was new about 15 years ago, there was a back-up battery in the Randall but that no longer seems to have enough guts to run the backlight for the display, so it may have failed. I'm now guessing that there may have been a mains failure some time ago which put the "logic" into the wrong state, where it stayed until my recent intervention.
My theories may be off beam, but I've been saved quite a bit of potentially messy work, and I thought others might benefit if they knew my experiences. (Of course, if I had flushed the system, I would have isolated the system for some of the time, and this presumably would have done the same reset, if that's what happened.)