er, fixed it... and I'm really sorry but I don't know which of two things I did fixed it, but I hope this helps someone else.
In a last ditch effort to rule out the impossible before a van came to take it away I thought it could (not) be:
* sidestand related, because the bike always runs fine on the sidestand with it up or down and I thought I remembered the sidestand safety feature leaves the instrumentation intact when it kills the engine if you try to ride off with it down.
* topple sensor related, because it hasn't toppled (where it was parked there wasn't room to the RH side for the bike to fall and no damage on the bike) and my bike has always started OK even after it has toppled and the topple kill sensor has activated (christmas period, 5.5 hour ride to keswick, arrived with full petrol and luggage at 3:00am and tried to do a u-turn on a hill)
I'd like to apologise in advance that I wasn't scientific about this and made two changes at once (I just didn't think it would be either of these things)...
I went and sat on the bike and with some deliberate speed brought it upright and slightly leaning to the other side to try and reset any tilt sensor (I have no idea of the mechanics behind the tilt/topple sensor on the DL, I was just guessing how it operates), and (sadly) at the same time I kicked the sidestand off.
The bike then lit up perfectly with the key, and started perfectly, as if nothing had happened. I turned it off, put it back on the sidestand and started it with it on the sidestand. No problems. Rode it home later with no drama.
so annoyingly:
* If originally I had just brought the bike upright and kicked in the sidestand in instead of starting it normally I might never have noticed there had been any issue
* If the van had come to take it away they would have put the sidestand in, wheeled it upright into the van, and there would have been no fault found... I would have been frustrated, the dealer would have been frustrated... and the fault never diagnosed.
I'm a bit stumped as to what happened but will ask dealer to take a look now that I can ride it to them and especially now that it's likely one of two things.
Thanks for all the suggestions folks, I will post any eventual conclusion here.