I am looking to buy some form of support for my sofa bed-not for the
mattress but for sofa mode.  In the UK please.

Hi Comeback,

My husband usually get a clean board. Then he cuts it to be a 
little less wide than the cushions and a couple of inches 
shorter than the area under the cushions. 

It's cheap, lasts practically forever, and easy to remove and
put behind or under the sofa when you open the bed. 

Any home repair supply shop or lumber shop will cut the
wood for you. 

Your TaxMama-ga

Was looking to but something but this might be a better solution-thanks

Here's a prefabricated product for firming up your sofa:

http://www.wdrake.com/shop/product.cfm?icat_item_num=41946