UPDATE
Unfortunately, after 14 blown fuses I've accepted defeat. I can't sort this to work on the F. Still works perfectly wired up as originally but following your suggestion, Clive, I swapped the black and the green wires. Casing no longer live, but motor doesn't operate.
Swapped them back and away it goes, intermittent-yes; speed 1 -yes; speed 2- yes. In desperation I tried isolating the motor and arms from the body, but it's not really possible, hence the mound of blown fuses.
Possibly something damaged in the motor is my best guess, but why work perfectly wired up wrong?
I've ordered a second hand pukka one.
EDIT
Here's a good tip! Don't switch the wipers on while holding the wiper motor in your hand. UNLESS you ensure your fingers won't get crushed by the mechanism. OOOOOOOOOOUCH!