Realised I haven't updated this thread.
To the person asking about heating, I had 2x 500w heaters. One at either end and also the pond is in my lounge, not a cold basement.
Anyway, the tropicals weren't working out, not a husbandry problem but my choice of fish just weren't suitable for top down viewing or the larger space. They were either too dark, too thin a profile or sat around doing nothing. I couldn't find anything that would suit. Rays seemed the obvious choice but after much research decided I didn't have the space. So I had a good hard think about what I wanted. Something active, something colourful, something of a decent size easy to be seen from above. The result... I have comet goldfish. I've got Yellows, Reds, Sarasa Orange & White and Sarasa Red & White. Had them 2 months now and very happy with them.