Close your eyes and imagine you’re in the supermarket. You’re in a rush, the kids are screaming and the shopping basket is digging into your arm. Only beans on toast for tea will do. But you’re faced with a choice.

Pay £3 for six tins or £2.69 for five. Which is the better deal? Go on, work it out. I’d hurry if I were you, the toddler has just realised he can reach the eggs.

Tricky, isn’t it? But leaving shoppers dazed and confused at bulk buy deals or slyly smaller pack sizes is what supermarkets have been reduced to in order to get one over on a generation of savvier shoppers, claimed Supershoppers (Channel 4, 6 June, 8.30pm).

Even Lidl is doing it. Smug discounter devotees would probably be appalled that a lighter Lidl Mars bar makes it far more expensive than at any of the multiples. In fact, of 11 brand names tested by the show 73% were far cheaper at its mainstream competition.

And these “sneaky tactics” don’t begin and end with grocery. Holland & Barrett is charging sex-starved shoppers £23 for Horny Goat Weed capsules to light a spark when it turns out the claims are based on nothing more than an urban myth about, well, horny goats.

As if that weren’t bad enough, it’s upping the dosage recommendations on scores of its costly own brand supplements compared with advice elsewhere. As a result, boosting omega-3 will cost you £120 per year at the health store, and only about 15 quid for a supermarket equivalent.

It seems we’ll all need to get a whole lot savvier. Or at least a whole lot better at maths.