If you're really trying to save money, try a British Heart Foundation 'Furniture & Electrical' Store. There's a list of them here:
http://www.bhf.org.uk...ture--electrical.aspx
Some people are horrified at the idea sleeping on a mattress which has previously been slept on by a stranger, but I can only only assume that they never stay in hotels! My current mattress is really comfy. It cost £15 from a charity sale a year ago.
If you'd rather buy new, Dreams have stores throughout the country:
http://www.dreams.co.uk/StoreLocator/
or you can buy online. (£79 for their cheapest double mattress):
http://www.dreams.co....ss/113-00221/product/
Chris