An acquaintance of mine once had luncheon with HRH The Duke of Edinburgh at Buckingham Palace. At the end of the meal Prince Philip escorted his guest to the private entrance of the palace where a Rolls-Royce limousine, resplendent in glossy back with deep burgundy sides, was parked, the door held open by the waiting chauffeur.

"Your car?" asked HRH.

"Yes," came the somewhat proud reply.

"Hmm," said the Prince. "Our colours."

Even though there may be no restrictions, I rather doubt it would be "good form" to wear either the Rothesay or Prince of Wales tartan.