Like you say if your lid is not very tight at the moment they may just be escaping as they hatch. Mine is tight so they are trapped until I open it.

Yeh, wouldn't bother putting any effort into getting rid them they will only be back in a week or two.

Thanks all...I will stop bothering myself!

I haven't opened my filter yet, its one that you flush out every week so I never need to open it.

I have just had the pump out though, and noticed all sorts of wriggly things in the pump cover and the pipes. I'm guessing its just more food for the fish

I had these within a couple of days of filling up my new pond. Not sure about them being a food source, but they are a benefit to the pond.