Thanks for the reply Robyn (by the way, your garden sounds idylic!) I bought a plastic heron yesterday, and another lily for them to hide under. I came in to watch the World Cup, went out an hour later, and the crystal clear pond was quite murky, I thought perhaps they had been digging in the new plant (even though I put stones on the top)
Anyways, I went out in another hour, and found one of my dogs staring down at a grass carp :( at least I think it was a grass carp:

The fish was just lay there dead on the cobbles. I washed it off and there is no sign of injury, other than part of its tail fin missing. Thing is, the pond was even murkier, but there was no damage, or even movement to the plants which line the sides. I know the dog hasn't been in, cos she hates water with a vengeance. Its a total mystery as to what happened.
Later, the fish were coming up happily for food, none of them looked spooked. Its so frustrating not knowing.
I'm in the UK so we don't get racoons, the only wildlife I see in the garden are small birds like blue tits, sparrows and robins.
The water is clearing this morning, and no more fatalities. If anyone can hazard a guess as to what happened, I would love to hear!