Thanks guys for all your suggestions. Robbo, that makes sense, it would explain why the valve came off so cleanly and without any traces of glue. Seems like this is a much more common problem than I thought.
When it happened to the first kite I tried to glue the valve back on but couldn't get it airtight. The kite doctor said that he couldn't fix it either, so I bought a new bladder. Not much change out of $200

I don't want to do that again.
I'll try my retailer again, failing that I will probably follow the suggestion of just patching over the hole left behind as I can easily live without the dump valve.
Holgs