And the reason it was mostly unrelated to my leaking deflate valve - as I mentioned in the other post it doesn't leak once the kite is inflated properly.
Someone was holding it for me while I pumped it up - was it was inflated I went to check it and it was fine, so I kept on rigging.
When I replay it in my head, I am pretty sure I neglected to velcro up the valve, so when I bounced it off the water there wasn't much to stop it blowing.

Live and learn - another one for the checklist.