This is why paper records are so important. All this tech/web records, they're not the same despite some mocking those who point out they aren't adequate.
I'm guessing but I'd suspect it O2 had records going further back they'd not have a reason to make them inaccessible. I wish you luck but my gut feeling is that 2 years is it, then it's deleted to release space for use by other data records.
Surely the courts know the situation though ? Or is this simply to back up a claim for your version of what occurred ?