I don't really do supermarket rage any more. After more than 20 minutes in the aisles, I tend to sink into a depressive trance, muttering a bit; the old days of cursing, rearranging the shelves and badgering other shoppers over the stupidity of their purchases are, I'm happy to say, over. You never win in supermarkets, it's like picking a fight with a bouncy castle. And while being banned from your local Tesco - an option once offered to me - might turn out to be a wonderful and life-enhancing experience, it would be pretty humiliating when you realised at 8pm that you'd forgotten to buy any salad and you had to go down there in dark glasses and a wig.

But when Tesco this week announced that it was banning Zimbabwean produce, you had to wonder at its PR department. Bet that'll really upset Robert Mugabe. He'll be going: “Oh, we have p***ed off Nelson Mandela and Tesco. I'd better retire, disband the war veterans and hand over to a pluralistic leader who can properly represent all Zimbabweans.” And Tesco - that caring, liberal, democracy-supporting Tesco - may well have put several hundred farmworkers in one of Africa's most hungry nations out of work.

I did lose it a bit in Marks & Spencer the other day. I needed new potatoes, lots of them, and the expensive ones were sold loose while the cheap ones were in plastic boxes, with big stickers saying “two for the price of one”. This got me twice over: first because I know that when supermarkets offer discounts on fresh produce like that, they usually make the farmer pick up much of the tab (if you want to challenge me on that one, M&S, I'm ready).

Also, I was on my bike. Wobbling home down the bus lane with a couple of kilos of potatoes suspended from each side of the handlebars was tolerable, but doing it with the plastic poking into the spokes and making machinegun noises all the way down Leith Walk was not.

So, on the far side of the till I started grumpily removing the potatoes from their boxes - and then I realised that this was way bigger than just me and my bicycle's aerodynamics. It was an anti-packaging protest. Cool! Consumers fight back! So I apologised profusely to the till guy - who looked like he'd seen it all before - and asked him to summon the manager. The shoppers behind me pursed their lips and sniffed in that special Edinburgh way that does make one consider moving back to London.

I apologised to them, too. “I just think it's terrible the way supermarkets inflict all this unnecessary plastic and paper on us, and then asks you, asks society, to deal with it,” I announced. They all turned and looked in the other direction.

The manager arrived, a large, Eastern European security man at each shoulder. One took me by the arms, the other silently gathered up all the loose potatoes. In an office, I was pushed to a kneeling position, my arms twisted hard up behind my shoulder blades. “Now, Mr Oh-so-Clever-and-Green, let's see how much you like our unpackaged potatoes,” snarled the manager, as golden Jersey Royals bounced around my knees. “Eat them!” The guards tittered as they forced my face towards the carpet.

Actually, that didn't happen. What did is that the manager shook my hand and thanked me for expressing my feelings at the till. “It's very useful to me, and my staff, to hear these views,” he said. “It's a reminder that we've just got to keep striving to do better on the packaging issue. That's something I think about every time I do my shopping. And we are trying - you see this week we're handing out free bags for life. But next week we'll be charging for them.”

I could feel the Morningside biddies nodding in satisfaction behind me. “You've been trained,” I said weakly. “Does this happen often?” The manager, smiled. “No, but when it does it's very welcome. Very welcome indeed.”

As I say, you never win in supermarkets.

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