They stopped the £15 for 1GB that doesn't expire plan at the end of last year. You still might be able to find the dongles around that are on that price plan. I saw one in WH Smith the other day.
It should mention the £15 for 1GB on the box and it needs to be on a price plan called "TopUp and Go Mobile Broadband"
I might dig mine out again and see if its still valid. Could be useful after all