I found this website (http://www.mysupermarket.co.uk) which allows you to compare the prices of your shopping trolley amongst the 4 online grocery stores (Asda, Tesco, Sainsbury and Ocado). Basically works like this:
1. Choose a store to start with
2. Find items and put them in your trolley
3. Site recommends alternative products to what you have in your trolley that could save you money
4. Before checking out, compare the prices for the same trolley (with substitutions) at each of the 3 other stores.
5. View the substitutions and choose others if you require.
6. Send your trolley to the store you choose and check out.
You even earn iPoints (www.ipoints.co.uk) for shopping through them
quidco pay £1 for registering then £4 for first shop :)