Thanks for that. I’m not surprised they couldn’t work out what was happening. I’m not sure I know why the evening maximum fare was only £4.80 because it’s usually £6.40 off-peak. I also think the touch on the validator at Farringdon may be confusing things because that should have been part of the main journey. Anyway, you are correct that you should have been capped, so just ask them to refund the rest down to the cap.
Moving forward, you don’t need to touch the validators when changing at Farringdon if you are making a PAYG journey. They only need to be used if the Thaneslink journey is using a paper ticket and you are starting/ending a PAYG journey at Farringdon. The signage is awful in that respect.
I hope you get it sorted out.
Edit. I’ve just noticed you spent 9 minutes at Farringdon, so probably went out of the gates, but if you are just changing and don’t need to go through both sets of gates then you don’t need to touch.
