Robert wrote in :

It is possible that you have a large number of planarians in the pond.
They are small, leech like, flatworms that will steadily munch their way
through frogspawn.

No idea, but I'll have a look. Thanks for a useful starting point.

These invertebrates arrived in our pond some years
ago and removed all the embryos from the clumps of spawn for a couple of
years. We now watch out for the spawn to appear, remove it to a couple
of large containers and return the tadpoles to the pond when they have
hatched - so that the newts have got something to eat :-)

That was last year's experience

But then I've got a soft spot for newts anyway