I noticed how dark green it is Louise, I wondered if you had fed it with Miracle Grow.
There's so many which look similar! Dyckia, Hectia,
Hechtia argentea looks similar, they change colour and are very variable. Young plants usually look quite different to mature ones..
http://www.scientific-web.com/...
http://www.google.co.uk/search...
There's a lot of species as well as hybrids.
http://fcbs.org/pictures/Hecht...
