All fixed! I spent the morning in the rain, and did the following:
Top boost hose off, relays at front of battery off, battery out & battery tray out.
I could see the problem already then - one of the cables (the one that moves when you move the gearstick forward & backwards) had pulled its threaded section straight out of the joint at the top of the gearbox.
So I threaded it back in, tightened the lock nut, then drilled straight down through it, tapped it, & popped a small M3 bolt down through it, to make sure it can't pull out again.
Put it all back together, and all is good - the gearchange is a bit tighter than before, so good job jobbed!
Oh, and for the record, the clutch slave cylinder on the 2.4 JTD's is external - right under the battery tray.