Tidied garage, cleaned out the fish pond filter again.
Funny the mrs thought I was nuts suggesting that the filter/pump we have is too small (read "you've spent enough on the pond..") but then this summer she sees the effect of:
a) not having a UV bulb working - pea soup.
b) the existing 4500 pressure filter being too small - cleaning out twice a week in summer (this is after the number of fish was halved)
Now the pond is crystal clear again. The plants are thriving, with the water cress really loving it in the veggie filter (lots of trapped matter in the roots and great water ammonia/nitrite/nitrate levels).