it certainly follows the growth pattern of so many echino's. If it has flowered that will help identify it but again there are now so many hybrids. Does it go outside for the summer ? cactus benefit from full sun before they come in for the winter, stops them getting too stretched. It's a nice branching little plant.

Nope. Looks like one of the Echinopsis hybrids, probably with E. chamaecereus as one of the parents--or grandparents.

It very well could be Echinocereus, although it is dificult to tell. As Bizzyb has said it would help it it has flowered.
They are generally smalish plants that produce low mounds, with spines arranged on ribs.

I still think it's an Echinopsis, bu I withdraw my suggestion of an E. chamaecereus ancestry. Under the etiolated upper growth, there is the adult form of E. oxygona, or one of it's near hybrids. The upper growth is typical of the juvenile growth of many species of Echinopsis, to which they tend to revert when deprived of light.

Thanks everyone. This plant has sat on my south facing windowsill for over twenty years and I am just getting round to finding out it's name. I put my orchids outside in the summer but not my cacti. I saved this plant from a group of cacti as it was being covered, maybe that stretched it, but it looks like natural growth. Will go to the botanic gardens and see if I can identify any cousins!!