I get everything back in place and work the mech by hand, and all ok,
so I put it all back together again, and now it plays in forward, plays in
reverse, but when you stop and try to play forwards again, it cuts out
because the trailing spool isn't going round. Why? because there is now a
small loop of tape in the cassette that's just long enough to not be taken
up before the autostop cuts in. And why is there a loop ? Because as the
mech comes to a stop, it just for the tiniest moment 'flicks' the take up
spool in the 'wrong' direction. And why does it do that ? **** knows is the
simple answer ...

If I understand your description correctly... it shouldn't matter whether it
flicks the take-up spool, because the "correct" way to design an autostop
system is to wait until the takeup hub stops turning. This is the way Sony's
WM-D6C -- and doubtless hundreds of other cassette decks -- work.