Do you ever have those days when you wonder how you learnt to walk and chew gum at the same time?
So, I figured out the spare would have to be removed.
Undid the big central retaining T bolt - eventually - and then set out to undo the two lock nuts holding the lower corners of the luggage rack to the back of the two fenders.
Just enough room to get a spanner in, but only to undo the nut 1/8th turn at a time.

Thinks bubble - I'm glad I'm not on the side of the M1 on a dark, rainy night . . . .
But, after twenty minutes of effort, I realise the locknut doesn't appear to be moving, and a little careful tactile use of fingertip reveals the whole thing is rotating - not just the locknut, which isn't budging at all.
Cursing Mr Morgan's design philosophy,

I then stand up and wonder how to get another spanner of socket on the dome head, and pull the luggage rack slightly upwards to see if there's any slack.
And . . . as you all probably know, the design philosophy reveals itself and the luggage rack lifts off

The rest is history - the four screws are tightened, the cobra is immobile again, the wire spare has been cleaned and inflated, and I've learnt a bit more about Morgans.

I'm beginning to wonder if a Haynes manual might be a good investment.