The Vulcan was always intended to have Olympus engines, but the first prototype had to fly with Avons (on August 30, 1952) because of delays in getting the Olympus ready in time. Olympus 100 engines were ready and fitted to the second prototype which first flew on September 3, 1953.

Many thanks PL. I knew there was something about the Vulcan and Avons but couldn’t remember what it was. You cleared it up nicely.