I can no longer manage the riding position (probably always been too big for me ) and would like to fit an adjustable stem riser for a more upright riding position. However steering column is approx 26mm dia. and handlebar approx 22mm dia. I can't find risers in these obsolete sizes. Is it the end of the road? Must I lose an old friend and buy a new bike? Any ideas most gratefully welcomed

This seems more like a mechanic/fitting question than this group. That being said, I think you answered your own questions with the provided facts. If you can't find replacement parts for your current bike and it does not fit then you need to consider replacement.

Little confused, I would like a better explanation and possibly a pic. Certainly there is an easy solution. Are we talking Stem or Bars?

Try to post some pics. A quick search of that bike turned up pics that looked like it has standard threadless steerer and bars.

As samat22 has fewer than 10 posts, he or she won't be able to post pictures. If you PM me a link to a photo album, I can help you with pictures. Short of that, the terminology used is confusing people.

What do you mean by "steering column"? Are you talking about the diameter of the stem, where it fits down into the steer tube at the headset? It could be an oversized quill stem for a 1-1/8" headset (where the stem is 1", or about 25.4mm). A standard 1" headset uses a 7/8" stem, or 22.2mm. The handlebar being about 22mm makes sense, as that's the standard grip size for a mountain bike/hybrid bike handlebar. If both the grip area of the handlebar and the clamp area of the handlebar are about 22mm, then it could be a BMW style bar.

On the other hand, if you're referring to a steer tube that is 26mm (outside diameter), your bike may use the East German or Austrian standard (see Sheldon Brown page below). Those use a 22.0mm stem, and a standard 1" quill stem (22.2mm) will usually not fit unless you sand a very small amount of material from the stem. You also might find luck with a circular file or hone to open the steer tube up just a little bit. With a 26mm outside diameter, it should have plenty of material for that.

https://www.sheldonbrown.com/cribsheet-headsets.html

Hi All, and thank you for your replies.
I found a local bike shop who had some old stock of the right diameters in the shed. They have fitted for me at a good price, and I am hoping to test on a longer ride when the rain stops. for anyone in a similar position, - needing obsolete parts - a long established shop may solve your problem where the internet can't.
Best wishes.