Probably the weed they sell in pet shops intended for fish tanks and bowls - I can't remember its name, but its got curly leaves and is usually sold in a clump of about 5 strands, anchored on a lead strip.

Thank you much appreciated barmy phil

Its called Laarosiphon (Elodea crispa) Bamboo. Its the one I have used for 14 years as it also provides shade. It can get quite big but I just remove some every spring to control it.
Not to be confused with Elodea canadenis which is very invasive.

Funny thing is, I remembered the 'crispa' bit... I had indoor goldfish for 17 years...

Thank you Drc726, I think that it is the Elodea Canadenis that we have in our pond and eventhough we remove a load ever year it has just taken over. I will remove all of it this time and will replant with the Crispa instead.Thanks again, Barmy Phil.

i did that and the crispa does do the sae just a month slower :o)