I caught the Oxford Tube to London recently and found myself chatting at the Thornhill park-and-ride to two of my fellow passengers. Clearly of the professional class, they were off to London to stay with a friend.

“Marvellous that we can park for up to 72 hours,” trilled the wife, “and park without having to pay a penny. Leaving the car here is a good way of avoiding Boris’s congestion charge. Our friend advises all his visitors to do it.”

I smelt a rat. Why should this gentleman be advising all his friends to use Thornhill park-and-ride? Did they all live in Oxford?

They did not. The two people I was talking to were in fact from Scotland. They had journeyed to Oxford from visiting a family member in Cheltenham.

This incident made clear at once a matter that had long puzzled me — why Thornhill is always so full. Word is obviously out in London that it’s a perfect parking lot for the capital.

Meanwhile, the council tax payers of Oxfordshire pick up the tab for this free service. Mad, I’d say.

In London, the Scots couple wished me a cheery farewell. They were extra pleased because, owing to an accident in Park Lane, we went in via Earl’s Court — stopping just round the corner from their mate’s flat.