Strange creature in wildlife pond

I live in the UK.

We've got two ponds in our garden. One is about 4' by 8' and the other is
about 3' by 3'.
The larger pond has some fish and also has some established frogs. Earlier
this year two large clumps of frog spawn appeared in the larger pond. After
hatching, the majority of tadpoles dissapeared, except for a few living in
amongst the weeds in one corner. I guess that most got eaten by the fish.

About 3 months ago we decided to build a small wildlife pond (With no fish).
This was after the frogspawan dissapeared.

The other day we noticed what looked like a large tadpole in the wildlife
pond (about 2" long), but didn't get a good look as it darted off to the
bottom of the pond. We have now briefly seen this a few times.
Some of the tadpoles have now left the fish pond and are now young frogs,
about 1/2" in size.

The large tadpole looking creature in the wildlife pond is a lot bigger than
the young frogs, so couldn't be related.
Anyone have an idea as to:
What is it?
How could it have got there?

T.I.A