If you're feeding a family or group of friends, then it pays to buy in bulk. That doesn't mean you have to go down the cash and carry for a comically large jar of Marmite — it can be as simple as going for the larger version of the same product, if you know you're going to eat it all by the use-by date.
Yoghurts are a good example of where this is an easy switch to make. You can always make individual-size portions by using glass jars (reuse sterilised jam jars for a thrifty option).
Comparison: A 170g pot of Fage Total 0% Fat Authentic Greek Yoghurt costs £1 (that's nearly £6 per kg) vs. a 1kg pot for £4.