Martyn – I had a similar issue in February regarding a ride in from Newark Airport to the Financial district in Manhattan.
I was quoted $48 when I booked – but when I checked the cost at my hotel a little later – I’d been charged $68+. (I’d been double charged tolls, plus the driver had charged back to a location in NJ). This driver also asked me for a tip!!!
There is a help option on the app which allows you to select a completed trip which can be found under trips and fares review.
You click on the trip cost you are questioning and select report an issue with this trip. You’re then given various options regarding refunds etc.
I hope this helps