Well this has been growing a coat of slimy blue-green algae over EVERYTHING for the best part of a year, but I've finally solved it.
It was too much light. I initially used the SMD LED bulbs from eBay. I tried 2.5w and 3w. Now I've gone down to 1.5w normal LED bulbs (the lowest I could find) and the plants are all growing like rockets and not a sniff of algae in sight!
So if you're thinking of building your own lighting rig, bang for the buck on normal kitchen GU10 LEDs is incredible. Just start low and work your way up!