Hello Glyn,

it's confusing, but You didn't read the manual very carefully!

Short excerpt:

The INHERIT attribute governs inheritance of grantable privileges (that is, access privileges for
database objects and role memberships). It does not apply to the special role attributes set by CREATE
ROLE and ALTER ROLE. For example, being a member of a role with CREATEDB privilege does not
immediately grant the ability to create databases, even if INHERIT is set; it would be necessary to
become that role via SET ROLE before creating a database.

bye...

Ludwig