Other cons rely on customer misconceptions, according to a recent Panorama TV documentary, The Truth About Supermarkets.

• Loose is always cheaper. In Morrisons some apples were £2.99 a kilo loose, £1.87 a kilo in a pack. However in ASDA loose was cheaper on red onions – 86p a kilo loose, £2.85 a kilo in a net.
• Buy more for less. In Sainsbury’s Cornish Pasties were £1.12 each, or at three for £5, rather more. This is a variation on the “bigger pack – better value” myth.

In fairness some special offers on regular sizes deeply undercut prices on larger sizes. ASDA are currently offering 500 gm jars of Dolmio sauce for £1, or £2 per kilo, alongside 750 gm Family Size jars for £2.98, or almost double the price at £3.96 a kilo.