Well, hire ar gone back and MG retrieved.
I couldn't open the fuel filler so couldn't top it up before taking it back. I followed the instructions in the handbook, locked it, unlocked it, and another guy on the adjacent pump had a go for me with no Joy.
I'm hoping the hire car company don't try and sting me for not filling it up, so I've used the app to suspend my credit card and took a video of the flap not opening, just in case. In any event, I did most of the motoring on electrons (it was a PHEV) so used hardly and fuel and the gauge was still showing full, so hopefully they won't make a thing out of it.