I AM not adept at getting a big shop. Actually, I’m not bad at gathering the items. I’m quite tall, so I can retrieve blackcurrant jam from the top shelf without having to go on my tip-toes. I feel sorry for short people, actually, who have to eat strawberry or Value Mixed- Fruit Jam.

The difficulty comes when I reach the checkout. I’m fine at emptying the trolley onto the conveyor belt, and these days I know where to place the baguette so that it doesn’t get lodged against one of the spare dividers and knock the cylinder of barbecue flavour Pringles onto the floor.

It’s the other end of the process which troubles me – the filling of bags. And, even then, it’s not the filling itself, it’s the opening of the bags. For I have a Teflon thumb.

Normally it causes me no pain. I can hold a pen, make a cup of tea and perform all of my morning and late evening ablutions, but ask me to pick up a five pence piece from a tiled floor or open a fresh supermarket carrier bag and you might as well ask me to trap moonlight in a Thermos flask.

I tear the first bag from its holder and hope that it’s pulled the next one open enough for me to jam a finger in. Usually it hasn’t. Then I attempt to rub the little flappy thing to make it open. That doesn’t happen.

Then I look at the handle for the tiny millimetre-width seam and eventually prise the bag open. While this is happening the groceries are piling up, as if in a Tesco version of The Sorcerer’s Apprentice. I feel like a contestant in The Generation Game: everybody around me is making pots with no fuss, while I’ve got clay all over my face and in my ears.

It is at this point that the assistant usually takes pity on me, and tears off eight wide-open bags with a single flick of the wrist. It is then the work of a moment to clear the backlog and look to all the world like a competent adult.

This is assuming, of course, that I am not being assisted by a charity bag packer. I would happily pay a charity bag packer not to pack my shopping. I have a system for filling bags, baked goods in one bag, fridge items in another, etc. The only system charity bag packers have is first come first served, eggs and yoghurt at the bottom, and tins on top of the soft baps.

This is what happens when children are given a job more suited to a grown-up. I have no objection to them doing jobs that adults are unable to do, for example, nipping up chimneys or repairing looms, but packing shopping bags is man’s work.

So it was with the intention of avoiding charity bag packers that I used the self-service check-out facility. And, to begin with, it all went swimmingly. The bag hung invitingly open on the rack. The items glided from trolley to carrier via the bar reader as if nature had intended it. And when the bag was full I tore it away.

But the next bag did not open. It just hung there, its white tongue lolling mockingly in my direction. I pulled it off. It was replaced on the rack by another obstinately closed bag. And as I struggled with my Teflon thumb to open the second bag, I became aware that there was a queue behind me. And I was flying without a co-pilot.

At this point, my body decided things were going far too well and directed a consignment of sweat to my index finger. I struggled some more. I even blew on the bag to try to part the tongue.

Then I heard it, an audible “tut” from the woman behind me. I had to cut the Gordian knot. I turned my back on the woman and surreptitiously pressed one knee on the pad. I have never been so glad to hear the words “Unexpected item in the bagging area.”

Within seconds, a supermarket employee was at my side. I expressed my bafflement, clearly there was nothing in the bagging area. He fiddled about with the machine as I whistled innocently.

Then, as an afterthought, he tore off half a dozen bags. If only I could use my Machiavellian powers for good.