No return call from the doctor this evening. However, my leg felt so much better that I'm reasonably confident that it's not the Achilles tendon, but a tear in the muscle a few inches higher up - which was what I thought at the time it happened. Walking is going to be a problem for some time to come, but at least I can put weight on my foot without pain, as long as I don't try to lift my heel.
I'll give it another day and then, when I see how it develops, get another doc's appointment to confirm/negate the original diagnosis, and ask for some recommended exercises for when it's recovering properly. As things are, I don't feel justified in sitting around in A & E for 3 or 4 hours (or more). It just doesn't seem serious enough at the moment to justify that.