OK the rules are:
We plan all the meals in advance and buy what we need for them
We buy as much Betterbuy (Morrisons cheapest brand) as we can get away with
We don't drink booze or juice or coke, just water and tea/coffee
We eat chicken occasionally but no other meat (too expensive)
We buy fruit/veg that is bruised/cheap, then parboil and freeze it for casseroles and pies etc
We make all meals from scratch and our own bread too
The secret is to know what not to skimp on - eg we always get good-quality stock cubes as they are a basis for tasty soups/casseroles
I have to add too, we will be topping up from the local shop for milk and bananas/apples, but we can often get bananas for free as they give away the old ones when the new lot are delivered - it's all about timing