You won'y get scalded and if you want to inspect the impeller you can switch everything off, undo the allen keys and pull it apart .
I manually set the diverter valve to 'MAN OPEN'. I powered the system back-on (the 'gas-fired water heating device' fired up) and I unscrewed the seep-cap. The CIRCULATOR spindle wasn't rotating!!! I manually rotated the spindle with a screwdriver (fortunately in the correct direction) and the motor fired into life!
I now have tepid heating (presumably it is simultaneously heating the water tank too).
Oddly enough, I had applied a 'percussive adjustment' to the motor at the very beginning but it made no difference.
Now I didn't expect the motor to run for very long but I had forgotten to turn the system thermostat back-up so after half a degree improvement the system shut-down. I dashed out to turn the thermostat up, the 'gas-fired water heating device' fired up immediately and guess what...the 'circulator' motor was still again. Once more I removed the seep-cap (this time I got a much more direct squirt of water than I was expecting!) and I 'kick' started the motor. Hey Presto! Central Heating again.
I shall have to turn the system off completely tonight because the circulatory device obviously can't manage entirely on its own.
Firstly, many thanks again for all your help so far.
Secondly, given this update I would be most grateful as to what you now think I should do;
(a) in the morning to get my immediate-needs heating back, and
(b) in the shorter term to maintain the entire system.