Iain & Gav: nice work, let me know if I can get involved, bugger being at the other end of the country....
That's what I'm using now. I should explain, I'm not exactly just "taking a dip", I am Kiting (often in the surf), and with regular impacts that can break ribs (not yet touch wood but close several times), I don't trust the bags or boxes long term. Several guys have had them leak after a big crash.
I know, I should act my age...

