Agree that this looks like a rounding issue rather than the annual price increase in line with RPI. If for no other reason, the increase isn't applied until April each year - so this years hasn't been applied yet. And as already suggested a 1p on £8.40 increase doesn't appear to fit any recent RPI rate.
Also agree that it's the principle and me being me I'd probably query it with CS, however whether 24p over the life of the contract is worth half an hour or more on the phone is a matter for you. Maybe worth a query with Live Chat given it's a low value credit, I'm sure they could sort it for you too.
I work for O2, however any advice or opinion given in this community should be considered my own and not necessarily representative of Telefónica UK Limited.
