Hi Craigandkim and welcome to the eForum
Thanks for your post here and sorry that this was not explained in full to you.
When you purchase a pay as you go modem you get an allowance (in this case 3GB) this is how much data you have available to you to use and this needs to be used within a month.
So when you topup with £15 pounds you buy yourself, 30days and 3GB worth of data.
I hope this makes sense. If you have any more questions and we'll be happy to help you further
Thanks
Wayne
eForum Team