Thanks to you all for your replies.

Unfortunately the taddies in this pond get eaten because of the fish

I have a second (fishless) pond with loads of frogspawn and no weed...

The first pond does get quite a bit of sunlight before the leaves get
on the trees and the water lilies. Unfortunately the previous people
were koi keepers and had virtually no other plants - I will be
improving this situation this Spring/Summer. (anyone recommend a good
online source for ordering water plants?)

I have scraped some weed out - quite difficult to get much out without
either falling in or going for a wade/swim in the pond! I have topped
up a few times with nasty tap water - had to because of leaks before I
disconnected the leaky water feature that was dribbling water all over
the flower bed next to the pond without me realising.

Had also unplugged the magnetic "aqua-mate" device attached to the
filter inlet before I read about this possibly helping to reduce weed
in the pond. Have now reconnected this and we'll see if it helps.

45 fish - some of whom are quite large do produce lots of "nutrients"
(what a polite word for fish poo!) so I have also cleaned out the
filter (totally disgusting job and I think the only job in the garden
for which I have ever worn rubber gloves!) which may help.

If anyone in striking distance of Devizes, Wilts would like some koi
(not valuable I am told but quite attractive in their own way) or some
large goldfish and what I think might be orfe (or they might just be
thin goldfish!) and can collect - then they are free to a good home!

Heather