I think you'd be well advised to camp next to Richard and then you can borrow anything you've forgotten. And, if possible, well away from Wolfhound person, at least unless you bring earplugs. ... which I was just going to add to the list anyway.

Also:
Reading glasses (for those with failing faculties, as mentioned above)

Lists of songs/tunes you like in case you can't think what to play or sing

Prompt cards/notes for songs if necessary (no discussion please - songbook thread is already dealing with this)

Manuscript paper if you are able and likely to write down a tune

J-cloths (infinitely useful)

"sports" type mineral-water bottle - handy to carry in the daytime, doesn't spill if you knock it over in the night.

small folding stool you can take in to sessions etc if seating is in short supply

Piece of bubble-wrap big enough to sit on (keeps you dry and insulated if you sit on damp ground)

We also take a 2-litre icecream box filled with lots of little items that may be useful - spare lilo bungs, pen, felt-tip marker, insulating tape, string, a length of that stripey hazard tape, matches, Swiss Army knife, etc etc.

And some sort of bag or small rucksack with room to carry some of the above around with you, as well as some picnic food, drinks etc.

Marje