OK Guys - just got an internet connection for the first time since Friday, so an update (I appreciate all your voices of concern!

)
I called back to the UK and asked our dealer to express to HD what my thoughts about this were (!). Shortly thereafter, I had an SMS from the UK HD rep, telling me that his colleague in Spain would be in touch. He called me, told me that he would await the report from the tech tent at the rally on my bike, then act accordingly.
The next day, the tech department - who had been so unhelpful on first contact - were falling over themselves to be helpful, so words must have been whispered in their ear.

They said no compression on rear cylinder, exactly as I had suspected and suggested to them. My guess will be a burnt valve or holed piston - though I doubt we'll ever know.
Harley provided me with an Ultra Classic from the rally to continue our holiday with, and have shipped my bike back to the UK with their bikes. It should be back at our dealers before I get home. They called me yesterday to let me know that they have ordered a new engine this time, so all should be well in the end...

This time the MoCo came through!

Jim