ooh, meglet just totally took the words out of my mouth about asking for surplus fruit/veg on freecycle, you see people offering windfall apples, plums, surplus runner beans etc, sometimes you see people offering unopened tins of baby milk or nappies that don't fit the baby anymore. In fact I'm going to have to do the same with a couple of packs of size 2's my little fatty has just suddenly outgrown.
Probably repeating suggestions here but when we run out of money (often) I do the following:
Spaghetti cheese: layers of cooked onion, garlic, dry snapped spaghetti, chopped tinned tomates with mixed dried herbs in, a bit of cheese and crunched up crisps on top, cooked for half an hour with foil or a lid on top, then another ten minutes with foil/lid off to make the top crispy. Very cheap.
Buy onions on sale, chop a load of them and freeze them in bags for use in all sorts of meals, same goes for all freezable veg.
Egg fried rice: fry an onion, add cooked cold rice, seasoned beaten egg, cooked frozen peas, soy sauce. Add any meat for a filling simple meal.
Pizza bread: Lightly toast thick bread, split rolls or even crumpets, spread with cheap pasta sauce or tomato puree, top with cheese and whatever you have hanging about such as leftover sweetcorn/ham/bacon/tuna/mushrooms. Also you could use a layer of picallili or tuna mixed with a little mayo/salad cream instead of tomato based sauce.
Baked spuds piled with beans mixed with tuna, or beans and cheese, or a mix of corned beef slices, cheese slices, and baked beans.
Asda own brand pasta n sauce/savoury rice are under 30p each and are tasty and filling. Add some frozen mixed veg or a tin of sweetcorn, and some fishfingers or sausages, very cheap meal.
Cheese and potato bake (Add bacon, sliced sausages or corned beef for a meaty version): Layer up sliced parboiled potatoes with chopped onion and grated cheese, some mixed herbs, salt and pepper. Pour over some milk, then top with cheese and bake until potatoes are soft and top is brown.
Peasants soup: Scrub veg instead of peeling, more tasty and goes further. Puree/mash if kids don't like skins. Fry onion, garlic, carrots, potatoes, add any other veg if wanted such as frozen mixed veg or aging peppers in back of fridge, tin of tomatoes, tin of butterbeans or other pulses, handful of split red lentils, couple of oxo cubes, salt and pepper.
Pancakes are good for quick cheap desserts, done simply with lemon and sugar. Also make simple biscuits using 6oz flour, 4oz butter, 2oz flour (cream butter and sugar then add flour, roll out and cut out, bake at 150c until golden) and decorate with a little icing and sprinkles, or a few jellytots. You could vary them with a handful of currants or a bit of cocoa/orange zest/spare cupboard ingredients.
Agree with whoever said keep in some flour, butter and sugar for emergency treats. Also a few tins of cheap peach slices and 20p packets of instant custard mix etc for simple crumbles, or even use the fruit underneath a basic cake mix for a quick pudding.
Stale bread is good for toast, and hard bread on discount at the supermarket can be brought back to life if brushed with milk and given 10 minutes in the oven. Serve with cheap stew for an easy meal.
Also no need to get rid of your pets: in the days before tinned dogfood, my gran used to boil a sheeps head for the dogs, mixed with the tablescraps, and rice, mash or oats, whatever. You could try asking your local butcher what cheap/leftover bits of meat they could give or sell you, but be careful of small bones.