p a pond in my back garden and filled it with tap water. Within a day or two there were these little things swimming about in there and i have no idea how they got there or what they are. They are very small (probably 1cm long) and they seem to swim sideways in a strange motion. There is now hundreds of them all across the surface of my pond. Would the frogs eat these do you know? Its a wildlife pond so i'm not going to put fish in it.

Thanks, Steve

maybe blood worms.

They sound like mosquito larvae to me. See http://www.fishpondinfo.com/mosquito.htm
If they have long tails, they would be rat-tailed maggots but I'm leaning towards mosquitoes.

frogs may eat them.

Sounds like those water striders.

you are going to have bugs in a natural pond, not circulating.