Mealplan.
I only buy what we need then. I check the cupboards first too for things left over and can usually make 3-4 meals from what I have in the cupboard/freezer.
I've also switched to the basic/value ranges, but I've swapped supermarkets from Tesco to Sainsbury's as oddly enough, they have a better basics quality basic range, and more choice too. Things like chopped toms, spaghetti in sauce etc etc are fab from the basic's range and make a meal much cheaper.
I probably spend about £130 a fortnight, but can eek this out another half a week, so on average I spend just over £50 a week of groceries (including cat food, and cleaning products).
I'll echo Michy with the slow cooker. It's a godsend making cheap tasty meals, and it's nearly effortless. I make a big stew from £5 worth of stewing meat (you can buy cheaper when you're slow cooking), a stew pack - £1 - and some potatoes and stock. All in all, the meal costs £7ish, this will last 2 evening meals and a soup for lunchtime for 2 adults and 1 3 year old. Same goes for chilli's, spag bols, chicken casserole, sausage casserole. All are cheap and nice.
We don't have big lunches and M is more of a snacky eater, but we will have things like spag on toast (basis's tin 14p and 4 slices of bread for the 2 of us about 15p per person) I do a batch of pancakes, or eggy bread, or we'll just have some houmous with crudites. All healthy but low on cost.
Also, think about food wastage - I try and waste as little food as possible, if there are things that I am constantly throwing away then I'll stop buying them, or get smaller quantities. I found a fab recipe for banana bread that I'll make once a week because I always have a few over ripe bananas and that serves as a great snack with a cuppa, and I know I've not thrown my money away (I know it's only pence really, but still...).