Notocactus leninghausii is my guess.

I agree - Notocactus leninghausii.
It is native to the southernmost province of Brazil - Rio Grande do Sul.
Keep it cold and dry in the winter. It will eventually form a column, and produces yellow flowers in summer time :)

Oh thank you so much, Tug & Hywel. My OH bought it from the co-op a few Christmases ago, it had a little Santa hat on & was called the Santa cactus ....... (I knew that couldn't be right, lol) so it's good to give it it's proper name.
It has grown quite a bit, there was only one column originally.

I'll show you a photo of mine. It's about 30 yrs old lol

Look forward to seeing it, Hywel.

I'll take a photo tomorrow :)