Could it possibly have been agricultural red diesel he was using? No fuel tax on it. Customs & Excise regularly target diesel engined road vehicles used by those with access to red diesel.
"...his car has so far lasted 30 years...."
It may work on a 30 year old engine (Land Rover maybe), but it is probably worth nothing more than it's weight as scrap, so the financial risk is small. I'm not sure I'd like to try it on a modern expensive car engine.