The T class fare rules clearly state 50 quid fee and any fare difference. This does NOT apply to retrospective MFU, but as you said, you changed the date. If this results in a fare difference (e.g. you bought in a fare sale and now the sale has ended), you have to pay it. If that's the case, I'd say it should have been picked up when you made the change: that's a reason to be displeased. But it's quite clear in the fare rules otherwise.