Although it felt fine mechanically, it was open-circuit all the time.
Apparently, LED Lenser don't supply parts and I couldn't find a suitable replacement in any of the usual electronic parts suppliers but I couldn't bring myself to write off such a beautiful piece of (over-)engineering. Time to see if it could be bodged.
I had an el-cheapo (but actually rather nice) 9-LED flashlight that I picked up at a conference a few years ago.
It had a latching end-cap switch that "felt" pretty similar. Could it be made to fit ? It turns out, the switch from it is an awful lot bigger...
On the other hand, its about the right height so perhaps it could be made to fit. With a little bit of help from a grinding wheel, it fitted...barely.
Solder the wires onto the top of the battery-pack...
...and its back to life !!
Its certainly not as water-tight as it was the day it left the factory in Germany (the end-cap doesn't screw down all the way) but on the other hand I never go swimming with it and it isn't gone to a land-fill, so I'm happy.
Now I just need to figure out what do to with the remains of the el-cheapo 9-LED flashlight that I trashed for this repair - I can't bring myself to throw that out either.