Well, there aren't any problems with goldfish in an aquarium. It appears that is what you are essentially planning (50 X 30X 20 cm). I guess a little air pump would be needed since there would be no other circulation. One can be bought for a few dollars.

Thanks Chas, I'll sort a small pump to suffice. I've rigged up a gauge to indicate how much the water level is falling each day. When that stops, that'll give me an idea where the leak might be, ie at the water line hopefully. But the leak could still be lower or at the bottom of the pond, but not showing on the gauge because the water pressure is not as great, therefore water not seeping away.

It is always possible that the leak is in your plumbing, if you have a waterfall or other feature anyway. You haven't mentioned how much water loss you have already observed. If it is not that much loss in height, it is possible that it is siphoning thru a fold in the liner after a rain overfilled the pond.

The loss is app 8" over a week & still falling albeit very slowly now. I feel it's definitely a puncture that's the problem.
I've now got a largish container (food quality) as a temporary home for the fish. I'm fortunate to have a well stocked & informed Koi retailer not 2mls away who have been very helpful with advice on such as filling the container with water from the pond & pulling a pond plant with roots to place in situ & also the type of food to give them & they'll be quite content.
Cheers.