thescullster expressed precisely :
Does anyone have any words of wisdom on methods to get the glass cover to
unscrew please?
And yes, according to the instruction manual I am turning the thing the right
way (although I have tried "tightening" it in an attempt to free it off).

Ours are oblong, either side, slide in to latch and yes they too jam. I
get a heavy block of wood and tap them with that, which always frees
them - though next time I intend smearing some high melting point
grease on the mating surfaces, to see if that helps.