I switched from O2 to another provider over 6 months ago and on a hunch logged back into my account to find that my account and my other halves were both in credit with O2.
It appears O2 don't automatically refund your account if you credit when you switch or end your account (or at least didn't in my case) but there is a page where you can request a refund. As the "churn rate" for O2 is pretty low at apparently 1% for contract customers this year it still means that around 150,000 people could be owed some money so if you were with O2, log back into their O2 site to check and if you are in credit after leaving get a refund using the link.
In my case it was £14 & £8 for the two accounts. Every little helps.
myO2 - http://www.o2.co.uk/myo2
refund link - https://www.o2.co.uk/apps/help/help?qid=1&q1=34