Thanks Lilly!
I have discovered a couple of things in the last few days...

Firstly, there is currently not a single UK-based renewable energy hosting company (that is,where the actual servers are based in the UK and the energy they use is renewable).

I also discovered that http://aiso.net is a very popular and highly regarded green hosting company. They run their servers from their own solar panels in the Californian desert and have a reliable record over 10 years. Many other companies offering 'solar hosting' on the Web are actually resellers of AISO.