Steve -
Thanks mate, closer inspection revealed exactly that, the knurled ring that adjusts the rebound is sloppy and I reckon that's whats happened. I've wound it back to 8 clicks from Max, which makes it just a bit harder than standard and I'll see what happens next time out but I think I'll also need to find a way to prevent it "self-adjusting". How come the things that should be self-adjusting, like cam-chains don't, and those that shouldn't, like this, do?
Reminds me of a song I heard once - "How come the girls you fall in love with never fancy you, how come the ones you dont', do?"