Hi Neil,
Thanks for the reply (and very much for the PM - will reply to that separately).

I will start a project thread, but want to do it in a considered way, so for the moment, here is what I have done to the old girl.
Checked base timing - appears correct at 16 degrees BTDC with ECU in service mode.
Checked plugs - seem fine, a little bit sooty, less so on one side after a drive.
Checked compression - all 185psi when the engine is hot (initial puff was over 150psi)
Removed the old Sigma alarm (just in case it's immobiliser circuit was interfering, plus it's siren was faulty)
I also bought a Gunson digital gastester, but it appears to be made of chocolate, all it did was warm up, melt the hot glue sticking the pulse pump to the back of the unit and fall apart. It didn't calibrate in air, so it has gone back to Amazon today and I will have another one next week.
Without the gastester, I thought 'can't make it any worse' and adjusted the CO pot a bit. I can't find any reference to say whether increasing the Voltage to the ECU richness the mixture but it does appear to run more smoothly at idle when it is a couple of turns clockwise from the anticlockwise end.
The car drives fine, so this seems to be a starting issue more than anything else. Hopefully I will meet up with lansledj from here next week if he can spare the time so I can borrow his Elanscan interface and then I will have some data to go at, which should at least point me in some direction. I am slowly (as I find the tools I haven't used for years, like timing light etc) working through the 'hard start' section of the EMU manual, but have that feeling it will be something stupidly simple. It is consistent too, doesn't matter whether it is warm or cold, starting without throttle, or with a bit. No firing for perhaps the first 20 compressions and then occasional fires, and the engine will run on perhaps two cylinders for a bit, get 3 and then 4 a few seconds later, puffing out strong petrol smells and black smoke. It doesn't seem to small all that much once it has had a run, but switch it off and it is just the same even if started up again straight away. One fault I diagnosed on a friend's Zafira was a faulty starter motor, which cranked over just fine, but pulled massive current spikes that kept resetting the ECU (dragging the supply Voltage down) and meant it never attempted to fire. Can't help but wonder if that is what I have going on here.
Thanks for the pointer on that screw - it appears to have a mangled O-ring stuck down the side of the screw, as if to stop it rattling loose in the bore, but perhaps it is a seal that is trying to escape. I will have a search for whichever manual details that (didn't see it in the EMU manual, which is the only one I have so far). As far a tamper proof cap goes, it did have a square of dried out gaffa tape stuck over it, but if that was an attempt at sealing, it was woeful.

. I have a good O-ring kit so when I know what to do with that screw (if it has an associated setup procedure) I will take it out and investigate. There seemed to be two drillings, the smaller one has some black goo in it and might be sealed.
Otherwise while delayed from sorting the starting problem I have been getting to the bottom of things like central locking, which was previously controlled by the alarm and the various rods were disconnected or had fallen off inside the drivers door. So that is now fixed and the car can for now be locked with the metal thing that people used to put into a slot in the door before remote controls... I am looking out for a new alarm, main choice parameters are a durable key fob and a low current demand in the set condition. One approach I have used before (on the Mk2 Astra) is to fit the Vauxhall stuff from a Vectra / Omega complete with deadlocks, which would be quite welcome on this car.
Of course everything I take apart reveals another job, so I will start a list once it has passed the MOT and give myself something to aim for.
I'd quite like to fit a wideband Lambda (HEGO) sensor and gauge as well, particularly if one with a matching bezel can be found to match the OE gauges in the dash.
Priorities:
1) MOT (expect the starting issue will also show up in high emissions, otherwise it should pass)
2) Alarm
2a)
3) Cambelt kit (it looks to have had one a couple of years ago, but didn't mention water pump or roller(s) so isn't urgent but I'd like to do it for peace of mind.
4) Full service - again lots already has been done, oil looks new, but will give me a known good starting point.
Ian.