Well Carole B, you were absolutley right about the trapped air, after digging down, and a little tunneling, the air started coming out slowly to start with, but then with considerable pressure, that did make me take a few very quick steps backwards, i don't think it was methane though.
I did have a big panic on for a moment though, when the water in the pond dropped by six inches, but soon realized in would do if the bulge had gone down.
It's awfully wet and mucky under the liner, so have put a pipe in as suggested, time will tell.
Thanks again for that answer.

Kind Regards, Old Herbaceous.
There's no fool like an old fool.