Some good staples to keep in are the following:
Passata - it's about 40p for a 500g carton and you can make it into curry, spaghetti bolognese, casserole, chilli, etc, etc.
Herbs - dried, fresh, whatever floats your boat
Tabasco sauce
Curry powders - various
Paprika and chilli powder
Quark
Mince - either meat or Quorn (I only use Quorn)
Chicken or Quorn chicken
Rice
Pasta
Peppers (you can get a mixed bag from ASDA from a good price)
Red onions
Frozen vegetables - try adding frozen veg to things you wouldn't usually think to add them to - i now put frozen broccoli in spaghetti bolognese and it's lovely
From those things you can make ooodles of really nice, quick, cheap meals. It's just trial and error really.