Yes, I did uncover the UK passenger side.
Sadly no positive cables, let alone a thick, battery cable-type one.
Sooo, I enlisted the help of a friend who is an auto electrician and who has helped on a couple of my older cars. I still had the battery conditioner attached and asked if it was possible to cut/splice the cables and get enough juice to the battery for me to quickly use the key fob to open the boot. I was concerned the cables were far too lightweight to take the current from the fully-charged new battery which had arrived. He said it was possible but the current would have to be "calmed" (he is French) by a lamp inserted into the circuit.
He asked why I couldn't open the boot manually, if the system permitted it. I showed him by inserting the metal part of the key fob, pushing and twisting it - as tried a couple of dozen times before - and the bl**dy thing popped open.
I couldn't then extract the key ... it was caught behind the boot lid. Took a bit of wiggling to get it to clear. Perhaps that was the original problem?
New battery installed, engine fired up and traction control, sunroof and windows reset ... and battery conditioner now attached!
The thing is, I am no clearer as to where the positive pick up point is when the manual boot procedure doesn't work. All the internet info say underneath passenger carpets. Difficult to see how it could be under the driver's side for rhd due to the pedals, but perhaps?
Anyway, thanks again to everyone who has helped me out with advice and moral support ... very much appreciated!