Tried to start the van yesterday after a week or so.
Key in ok, glow plugs light and go out. Turn to III to start, nothing.
Fiddled with footbreak, Park, steering, to no effect.
After a couple more attempts, I find that the dashboard lights are all staying on, even when I turn to 0 and remove the key completely. There is no spring resistance felt between II and III any more.
Disengaging the battery neg lead then reconnecting (with the key out) causes all the dash lights to come on and the glow plugs to light, just as if it the key had been left in, in position II.
WD40 - no good. Tapping gently with a hammer (as one does

Undid cowling over ignition, nothing to be seen there - the "must be in Park" inhibitor working exactly as it should, and interacting with the key properly in every respect (e.g. preventing key removal at the right time, not allowing gear shift unless key in and turned, etc).
All the evidence points to a mechanical failure in the ignition barrel - possibly some kind of jam or break to whatever spring/rotation system is in there. Causing it to not turn the starter motor, nor to properly turn off any more.
Any ideas how I could deal with that, short of sourcing a full replacement (which, for me, means getting a professional to come and do it)? The ignition barrel doesn't look like something that could be readily disassembled or maintained.