How can I use my Vodafone Bonus Bank credit?

I have recently put a top up on my Vodafone Pay as you go phone. After completing the transaction I got a text saying that i have �15 in my Bonus Bank credit. What I would like to no it how I can use it, ether as credit or towards something else. I have looked on the Vodafone website but I couldn't find anything about it, so if there are any links then that would be very useful. Thanks for helping.