Hi Lee
This isn't simply a gratuitous plug - but, if you've not already seen it, you might find some of the advice on making clean water ponds that we produce at Pond Conservation useful.
Have a look at this blog post, and also the summary leaflet on our website.
How to make a really good wildlife pond (3): the pictures The Garden Pond Blog http://www.pondconservation.org.uk/N..._1July2009.pdf
Making a clean water pond will contibute to the UK Biodiversity Action Plan target for high quality wildlife ponds; it'll also mean you have less management problems with filamentous algae, duckweed and green soupy water.
Best wishes
Jeremy Biggs