Well, I'm doing it all a bit arse about face. I've rested it for a week, and now at least it's pain-free when walking (still stiff after long inactivity though). Having just watched the video below I thought I'd give it some ice, too.
I'm hoping three weeks of TLC will see it right. Otherwise we'll be looking at a bare-bones trot-round effort, or if it keeps coming back, DNS. Let's try and keep positive though.
Interesting side issue on the achilles is that, as it's a tendon, it receives very little blood flow, so takes much longer to heal than a muscle.