Terry Pinnell:
Before my last trip to the USA I obtained an international driving
licence, thinking I'd need it for car hire.

Since the so-called international license is just a translation of
your own license into other languages, you were mistaken.

But I don't recall Enterprise asking to see it. For another trip
shortly I've hired a car from Avis and read on the confirmation
that I need just a 'Non-US Passport and a Non-US Driver's License'.

When/where is an international licence needed please?

In some countries that don't speak English (this being the language on
your UK license). I don't know which ones.
--
Mark Brader, Toronto | "You don't SIT IN the traffic jam;
| you ARE the traffic jam." -- Werner Icking