Maybe Bananamal is being a little harsh, but I can understand that point of view.
Your contract is your responsibility. It is up to you to read all the terms and conditions of your contract and your bolt-ons.
As for O2 calling at the end of a minimum period to offer an upgrade I can understand why they may not do that any more. It's more cost effective for them to keep you on the same contract with the same phone, until you decide you want to upgrade or cancel the contract.
In 4½ years with O2 I've ceratainly never been contacted near the end of my minimum period.
Maybe I'm a little biased because I check my bill online every few days to make sure that there are no charges going on that I was unaware of, so I can deal with any problem swiftly.
I can understand that in the days when we just received a paper bill every month, you could get a nasty suprise, but when there's an online bill checking facility, it just takes a couple of minutes to log in and check recent charges every few days.
While I have some sympathy, as it's fine us saying you should keep an eye on your account, this is all in hindsight and doesn't help you now.
You never know, though, O2 CS may be able to help if you sweet-talk them. Worth a try.
SV