I offer a cautionary story today concerning travel on the London Underground. It arises from a trip I made last month to see the (wonderful) Inventing Impressionism exhibition in the Sainsbury Wing of the National Gallery.

Travel from Oxford to London was by train, life being too short for the X90 or Oxford Tube coaches early in the day or when the evening rush begins. From Paddington to Trafalgar Square was a 10-minute trip on the Bakerloo line, payment made with my Oyster card.

When the time came for my return tube ride, however, I discovered there was insufficient credit on the Oyster to open the barriers. The only means of ‘topping up’ at this station is by machine, credit or debit card payment only. I just had cash.

No matter, there was plenty of time before my pre-booked First Great Western train home. I would use the 23 bus, on which, with my old person’s pass, I travel free.

Reaching Paddington, I still had 15 minutes spare, so went down into the tube station (where real people sell tickets – but not for much longer) to top up my Oyster.

In a transaction at the counter, I handed over £20 and the card and got back the card. I asked for a receipt, was seemingly not heard the first time, so asked again. One was presented. I shoved it in my wallet, unlooked at.

Some days later, inspecting the wallet’s contents, I was surprised to find that the receipt credited me with spending £2 on my ‘top up’, with £18 change given. Transport for London’s customer services team (once I tracked them down – not easy) sorted the matter out and restored my £18, with much apology I have a view about all this which I will not share. Just remember to ask for a receipt.