Is there any way i can add more data to my contract?

I'm on a vodafone pay monthly iPhone 4s contract.
Unlimited mins/texts, 250mb data.
Is there any way i can add another 250mb or something? I know Vodafone do a "Add 1gb for a extra 10 pounds" but i was wondering if they do smaller data add ons?

One wonders why you don't contact them and ask.