Found this site called My Supermarket.http://www.mysupermarket.co.uk/Home.aspx
On home page you are given the option of which supermarket to shop at.
As you shop you are advised how you could save on items e.g. if item is bogof will let you know or alternative purchase
You have to register to complete shopping at supermarket of your choice but only submit payment when you get to actual supermarket site, so they don't have your card details.
Just did a comparison check on 11 items & got the following results
Tesco - £27.04
ASDA - £33.66
Sainsbury - £32.87
ocado - £37.04
Checked the Terms & Conditions & it states at present service is free but they may charge for the use of it in future. Probably build up a clientele then charge.
I haven't gone through to supermarket (Tesco), as yet, but reckon it would be well worth having a look at it, as you can do a dummy run to see your savings in each supermarket.