Have you tried using Mysupermarket.com? I find it much easier to keep track on buying exactly what I need, the 'favourites' really help here. I there for don't get sucked into the offers that I don't need when I buy online.
With Mysupermarket.com, it will automatically show you where you can save money in your trolley ie: you've bought Napolina tomatoes did you realise that the chopped ones are on offer? Also shows you a 'downshift option' from Martin Lewis.
Yes you do pay for the delivery £4.50 - £6.00, but you easily save this on all the offers that are useful for you, don't forget you save on petrol too. Also scour HOTUKDEALS for vouchers which give you money off, I've found vouchers some weeks which have given me up to £20 off my shop by simply putting in a code! Love it when that happens!
I would also say definitely plan your meals at the beginning of the week when you do your weekly shop, therefore you only buy what you need. You could even plan your meals around whats on offer.
Another idea is once a month, just buy the basics, milk, bread, pasta etc and try and live off the freezer and storecupboard. Finishing off all the things at the back of the freezer and cupboard. You'll be surprised how many meals you'll get out of the odd ends!