Stephen wrote:

Some of the goldfish we lost where about 8inches long,
are they not too big for a grass snake to take?

I imagine they would be too big. Mind you, a female grass snake can grow to
over a metre long, so might be able to handle an 8-incher on a good day!

However, I think it's much more likely that a heron was the culprit.

A nylon net and/or trip wires are probably your best answer.

Incidentally, is your pond quite shallow at the edges? A deep pond deters
herons as they don't like to get their trousers wet. They like to wade into
shallow water.

I built a pond that had steep sides and no shallow areas. I only ever lost
fish to grass snakes.

--
Interloper