Most garden centres or large DIY stores with a garden department should stock fleece. Hope you manage to find some. There are also websites that will be happy to post it out to you. Just 'Google' for garden fleece and you will be able to compare prices.
Kays Horticulture are very good, reasonably priced also. I believe they have a website...hang on I will go check.... http://www.kaysdiscountgarden.co.uk/ Just stick fleece in their search box and up it all comes.
