yes it is a Mammillaria.. sorry, don't know which

Names can be so hard, and start so many arguments. Here is a gallery of decent Mamm photos to while away the hours. Your plant is a fairly common one, it also resides in my collection. (Maybe M. zeilmanniana?)
http://www.cactuspro.com/photos/recherche/mammillaria--1.html

I believe that's Mammillaria boscana.

Planto

Thanks everybody, I've looked at pictures of the kinds you suggested and zeilmenniana looks quite plausible, I think I'll go with it. Thanks for the advice, however unimportant the name of the plant may seem...
Z.