£400 a month for everything, including milkman's bill and dc's packed lunches. That's for 2 adults and 3 school-age dc.
It's really not that hard -- I could do it for less if we had to, but below a certain level it does take more effort. Concentrate on prices for the items you buy a lot of, which in the frogpond are things like breakfast cereal, loo paper, cheese and yoghurts and the expensive items like meat, booze and washing powder.
I shop alternate weeks at Lidl and Morrisons, and the price difference on the products I've listed above between those shops and somewhere like Sainsburys (never mind Waitrose) is staggering.
Eg Lidl's own breakfast cereal (F&F or Shreddies for eg.) is between £1 and £1.49, whereas you can easily pay £2.50+ for an equivalent packet of Kellogs branded cereal. Ditto washing powder, loo paper, yoghurts etc etc.
Buy basic brand of everything unless it's proven to be disgusting (and Lidl's stuff is generally nicer than the Sainsbo's basic range ime).
You also need to plan your meals, make sure you use all left-overs and use ingredients like lentils, pasta and veg to bulk out the expensive ingredients, eg. stews rather than whole cuts of meat. Be aware of which products are expensive for the number of meals you get out of them, eg. chicken breasts are convenient, but v. v. expensive compared with buying a whole chicken. Ditto things like pre-grated cheese, or processed products like babybel.