We are a family of 4 (me, DH, DS4 & DS2) plus cat (so slightly larger family, but small children) & we spend between £100 & £200 per month on groceries including cat food, cleaning products, toiletries, nappies etc. The amount spent varies depending on what I already have in - so this month, the freezers are full with meat & fish, plus we have plenty of pasta, rice & drinks, so we should really only spend on fruit & veg, milk, yoghurts etc, so it will be a low spend month. I don't do a regular shop, just pop in when we need something.
The best way I have found of keeping costs down is to go to the supermarket (Tesco for me as is closest) in the evening, a couple of times a month, at around 7-8pm for our local store. This is when meat/fish/cooked meats etc get marked down to 20-25% of their original price, fruit & veg & bakery bits usually go for 10% of their original price. Most of this can then be frozen & constitutes the bulk of my food shopping for a month & so I only have to to up for items mentioned above.
It can be a pain to do it this way, especially if I have to take 2 young children with me, but I think it's worth it as it saves me hundreds of pounds a month.
Another helpful thing about doing things this way is that I rarely spend more than £20-£30 in one go, so I always get the till spits for money off next shop or more points if you spend £20, so then I can buy any toiletries etc with that.
I have also recently discovered Farm Foods for basics - milk, 2 x 4pts for £1.60, large sack of pots for £3, bread 2 for £1.50 (I think - good if I can't get it reduced!) and so on.