You can of course use your old potatoes for a new crop. The reason why people do not generally is that 'seed' potatoes are guaranteed virus free and saved ones may contain virus picked up in the normal course of growing.
Also, it is not that easy to keep stored potaotes in good condition until it is time to plant them again.
Do not forget that shop potatoes are treated to stop them shooting in store.

The seed potatoes are grown in areas where they are free of potato blight like the Arran Islands which are in the flow of the Atlantic winds.

If you have grown your own from seed potatoes originally then you would know if they were blight free, then it would jut be a matter of over wintering them in a dark place till next spring.

I didnt know the shop ones had been messed with so no good doing that then.

May get advice later on and try keeping some of my own crop at the end of autumn if they are blight free just to see if it works. I can always order more seed ones the following year if it doesnt. Of course that is assuming they grow in the first place lol.

Thanks for the answer.

Gardeners who grow heritage potatoes generaly start with just one or two of a variety and then save some smaller ones to bulk up their stock. In more southern parts this can be chancy because of the possibility of them getting diseases or virus - this is not always obvious at the time of harvesting.
I am lucky in that I live in seed potatoe growing country. My neighbouring farmer over the fence grows them in the field every few years. Even then, the cropping fields are well separated and only used for potatoes about every five years.
By all means Cb, try saving your own seed, don't let us put you off, but I would suggest that you only do this every alternate year, then buy fresh.

unless they are organic the potatoes in shops are irradiated, as are many veggies, to stop them sprouting/going bad.

I personally think it is well worth while buying fresh seed potatoes each year. We tend to change the varieties we grow each year as well.

well looking at the back of my veggie cupboard i have that many sprouting i could plant a field. I am surprised that they would be irradiated but if thats the case then mine must have missed it :o)

I have grown them before too and they are fine.

I wonder how they get away with it in Jersey where they keep twenty per cent of the crop for the following year and continually grow in the same ground.

Possibly they have never had any potatoe viruses on Jersey and keep it that way by only using their own potatoes? Just a thought.

lol
when we had up to 15 at home my wife would be in a hurry to peel the spuds and i used to just throw the peelings into the ground and weed out the sprouting ones that looked the weakest
some of her (peelings) were substantial

According to Wiley Interscience they do suffer from blight they spray the fields and fumigate them when packaged. I think I've just gone 'off' Jersey Royals.

Hmm, thank you for checking that out, Wagger. I guess that this applies to the nice early 'earlies' in the Supermarkets. I will try to remember to give potatoes grown on Jersey a miss, as well.

You would never eat potatoes again if you saw what they spray the fileds round us with before harvesting. Sulphuric acid is what it says on the tanker and the fields are barred and lock gated for 10 days afterwards.

The sulphuric acid is to burn off the haulms so that they don't get in the way of the large modern harvesting machines. In the 'good' old days the farmer would drive round with a 'spinner' which would throw the potatoes up onto the surface of the soil and an army of school children on half term (tattie picking) holoiday would pick them up.

The sulphur also acts as a fungicide to kill off any blight spores in the soil or on the potatoes. We went and gleaned a few after they had finished harvesting, but the potatoes were unusable because of the smell of sulphur on them.

I didn't know that but it makes sense.

We now only grow our own or buy organic after finding out they spray the crops with roundup before harvesting.
Even the council contractors when we asked them not to spray our verge because of our goats and pony said that the stand down period on roundup is 48 hours.