The situation used to be as follows: any additional data you bought to keep yourself online after depleting your allowance would expire at the end of that monthly cycle. As of today, though, any unused bolt-on data won't expire, and instead will rollover into next month's cap. So, if you get 2GB of data with your contract and you buy an additional gig but only use half of that before the end of the month, you'll get a 2.5GB allowance the following month. It's a one-time deal, though: all data, including any that's rolled over, will expire at the end of that monthly cycle. It's nothing like iD's rollover scheme then, but it's better than squat, especially if you consistently go over your data limit and top up with bolt-ons.
O2 also announced today that new and upgrading customers can now take advantage of increased data caps on a selection of pay-monthly, data-only and pay-as-you-go plans. Some tariffs have seen their data caps grow by as much as 50 percent, and you can check out O2's press release for a full breakdown of the affected plans. Bonus data is only available on these for a limited time, though, with offers expiring towards the end of March.