Thanks @N1Rotate, very useful. So it seems like this is a distinct possibility.

I separately note that there is zero tolerance for any trace of alcohol on and in a driver, so although the price to hire a car is good value, actually getting a cab is going to be a much better option with the temptations that are likely to encourage me to have a drink or two on the flight from BKK.

Any other tips or suggestions welcomed.... with the QR transit visa I am sure this could be an increasingly popular option for travellers on transit when the city tours are not running or oversubscribed, especially outside of the heat of the day.