Feck me, job done. I gave it one last try before disconnecting the shocks and it virtually fell into place.
What I did was: put the boot on the swinging arm side first, making sure that the edge of the boot went under the two clips at the top and bottom of the swinging arm mouth, then put the screw clip (the one like a strap) around the boot and tighten.
Then the fun bit, which this time turned out to be a complete non-event when it finally went into place:
I then put the boot on the right-hand side of the g'box boss and hoiked it with a screwdriver, a pointy-nosed pair of pliers, and an assortment of bent wires as far as possible top-and-bottom, then go around to the other side and simply carried on hoiking it around the rest of the boss.
Now that last bit seems SO easy that there seems to be little point in posting it,

FFS I must have tried it exactly like that the last time, but the truth is that the last time it took most of the day, and initially it looked as if it was going to be every bit as bloody-minded today as well. As it turned out this last attempt took less than five minutes, so I'll be following this little reminder exactly myself the next time I have to do the job, with the certainty that it did work once.
Thank you both for your replies, I may well get a genuine BMW boot next time.
