Pay As You Go

Just read through the PAYG details but still a little confused. I understand the Sim Only Deal and paying up front for the service. So say I pay £6 for the basic sim deal, at the end of the month is the data/calls still available if I don't renew the deal? Does it roll over every month? I really want a extra SIM for my current deal because for all the will in the world I never use my data or calls, but I understand that this isn't available.

So, in a nutshell... If I buy a SIM only deal for £6, does it roll over month after month?

Ta