Gill
You should be able to purchase general hydroponics ferts. Here is the UK website:
http://gb.eurohydro.com/
I have used their flora series in the greenhouse this winter-the plants that are on this thread are grown with the flora series, supplemented by a little botanicare 6-6-4 with calcium, and a tablespoon of epoma lime. There is also a floranova nova product on the same website that is extremely concentrated. I just bought some yesterday to try.