OK, I know that this is a very common Mammillaria in collections and I ought to recognise it. But could anyone confirm that this is what I think it is (i.e. M. prolifera) and possibly what it's sub-species name is (if it has one). I've seen a picture of a similar plant called M. prolifera ssp. texana.

I've not had the plant long (I rescued it from a local nursery) so I haven't seen it flower/fruit yet.

Yes, Simon, it is M. prolifera, but I don't think that it can definitely be named as ssp. texana. According to Pilbeam, there are two variants of texana, one with brown tipped spines and the other with yellow spines. However, ssp. prolifera itself also has pale yellow spines.
Which is yours? to be honest I can't really tell the difference, so I can't say. Maybe others can help resolve this.