Make a note of anything you want to buy and check out the price in your normal supermarket (you can check prices on supermarket.com). Then when you're near a Lildl/Aldi/Poundland/Wilko or Savers (the latter three sell cleaning products and toiletries cheaper than most supermarkets) buy the products in there if they're cheaper. So either buy the cheapest place possible or buy value, basics - they are just as good.
I make veggie spa bol, fry half or whole onion, add garlic, basics, value or Lidl chopped tomatoes, half packet quorn, I like a little tomato puree, herbs and a little hot water for extra moisture - this gives us three portions, although you could really stretch it out if you mixed it in with lots of value pasta.
If you've got curry sauce, add any veggies you can muster - carrot, broccoli, value peas - doesn't have to be packed with them and I find chickpeas work well with something like this, then plenty of rice. If you buy a large can of chickpeas (cheaper to buy dried if you don't mind cooking) and can't use all of them in one go, I've been known to add half a can to chopped tomatoe, rice and add curry powder. Onion works well adding flavour to a lot of things.
If you chop the chicken up in smaller pieces and add it to a stew, casserole, other dish, it will look more, ie two pieces could serve three of you.
A couple of veggie or meat sausages can be added to a tomato based sauce (fried onion, can tomatoes and any flavourings you have) and mixed in with pasta. You only need a couple chopped up small to add something for 2/3 of you.
If you have flour you could make a batter and odd chopped up sausages to that, ie toad in the whole. If you've got loads of batter, you'll get away with far less veg.
New potatoes look more on the plate if you chop them in half first.
We've had times when money is tight and toast is a great filler. If you look in your cupboards you've probably got things like jam, peanut butter, marmite and other spreads that need using up.
Goes without saying, don't waste anything. Even if leftovers don't go with another meal, have them as a snack or with a piece of bread for lunch yourself.
Think about how you can get what you want cheaper, ie you fancy a shop bought cake with coffee - value biscuits will be cheaper and still give you a treat.
I think Lidl have a cheese offer on this week of £1.39 for a large block by the way. Look out for packs of fruit for £1 - I got 7 apples last week. Value frozen veg is definitely cheaper and things like peas, sweetcorn and broccoli are great for adding small amounts to dishes.