Thanks so much everybody for all your suggestions. I had a great time - if very nerve wracking for the 24 hours beforehand: once we actually got there, about 2 hours before lift off, it was fine.
I wasn't expecting to find an audience, but was aware all the time of people clapping and sometimes dancing, so I was playing a few things like Pugwash on the spur of the moment. That one worked because I could hear a voice quite close saying "Captain Pugwash" in a sort of surprised recognition. It did mean that the planned repertoire went out of the window a bit.
Plenty more tunes to play like Rodney's Glory, which I really like, but I ran out of time - particularly when the dancers got going. It was a really good feeling playing high up like that - I wonder if there are any other plinths or pedestals available. I could even become the first concertina-playing stylite!
The only slight worry on the night was that the concertina occasionally felt as if it was slipping off my lap - a weird, and completely false, illusion caused when I looked downwards and the ground wasn't all there. Though that wasn't the only reason for the odd bum note.
We've got just got home so I haven't been able to contact you all before, but all your ideas and the immediate support made it much easier to complete this little project and I am really, really grateful. As one of the other plinthers has already said: I haven't quite come down to earth yet!
Dave