Davy. Yes, I have found that one now! I pulled it out but there was nothing to see, so refitted it. I took the ‘dry’ one out; behind the r/h cylinder, removed the flat aluminium washer, re-fitted and cranked it, but it didn’t start. That would have reduced the sensor to flywheel clearance a little. Having said that, it ran with the flat washer and ‘O’ ring in before, so probably not that. The battery was a bit low, which didn’t help, so it has been on charge overnight.
Cheers Ian
I will try that this morning! It does have hazards.
Thanks Ian
RESULT! 
I removed the plastic injector covers, to ensure they were not interfering with the choke linkage. I then removed the 6 relays in the block, 2 of which I had replaced with the used ones I got from Gutsibits, and sprayed them with contact spray. I sprayed the fuses, and the electrical connections under the left-hand side panel, refitted the relays, and tried it. Fired right up with no throttle, cleared its throat cleaning up the plugs; although I had given them a brush before refitting them; and settled down to a tick over with the choke now off. Refitted the injector covers, tried again, running nicely.
So what was it? I can’t be sure, specifically, which is a bit annoying 
I cannot thank you all enough for your input; many thanks.
It still has a couple of issues. The starter button only works when it feels like it, needing several stabs before it will work. I guess that is just a case of removing and cleaning it up.
When riding on the IoM, and on the way back, it suffered from a hot start issue. Once it just would not fire, fortunately there was a very steep side lane by where we stopped, and it bump started ok. Needs resolving though.
Then there is the issue of a heavy-handed PO who has sheared off the heads of both the gearbox and rear bevel drive drain plugs 
…
Regards Ian
Right so bad electrical connections then.
“I hate being right all the time” (quote borrowed from Jurassic Park).
Suspect that is also the issue with the starter button.
\
Mike.
yes, I am sure you are right. Likewise the starter button, as you say.
I will see if the hot start issue continues. If so I may start another thread on it. 
Cheers. Ian
Some joy in this wintery gloom!! Glad to read of your progress! Does the starter solenoid click when you press the button unsuccessfully (e.g. a start issue) or not (see earlier comments re cleaning up electrickery)?
Good question, I interpreted it as meaning press button and sweet diddly-squat happens.
I had this with a horn button, which was part of a left handlebar cluster off of a Honda Superdream, and was also intermittent; (MUCH) later I discovered a connector to wire crimp inside its plug block had gone loose. Which explained why, hitherto, no amount of squirting with any kind of “magic spray” was a permanent cure.
The whirring of the fuel pump drowns out any clicks from relays, so I can’t be sure. However, I understand this is a common problem with Guzzis, caused by the fact that the factory wired the starter circuit through the switch, which it is not designed for. The fix is to alter the wiring to the starter relay, I believe.
Before I do any mods, I will first cleaner up all contacts I can find, starter button, switch, starter solenoid etc, and fit all new relays when they arrive, and fuses.
I will remove the pump fuse to shut it up, and listen out for a click from the relay. My hearing is not what it was…
Yup!
This is a common issue (not starting on the switch) with a well-documented and simple fix. Try Greg Benders’s site (thisoldtractor) for the solution. The wiring remedy works a treat. (Proven on a '93 Spada…)
If the fuel pump relay is “upstream” of the relay, removing it will take out relay too. No need to try and hear the clicks - should be able to feel them through finger tips. You could take them all out, put them back one at a time and see what wakes up!?