Thanks to all who have contributed. Armed with the magic term 'SRPP' I searched again on the net and came up with this detailed explanation (see link below). It doesn't quite meet my requirement of simplicity but it does cover the ground, offering equivalent circuit descriptions and the development of the original idea.
So, from this thread I have again learned something I'd never heard of before, which goes to prove that one is never too old to learn (or would that be better to say that one can't teach an old dog new tricks?). Either way, it has proved an interesting if winding path to follow.
I won't, however, be including it in any future amplifier build.
http://www.valvewizard.co.uk/SRPP_Blencowe.pdf
Tony