Thanks all, you've given me a lot of food for thought!
I think, as I'm still relatively young, that the pain must be caused by tight calf muscles rather than an overuse injury and I've found some strengthening exercises the calves and achilles. Fingers crossed that they heal up in a couple of weeks.
the 3.3s ballistecs were most likely not helping though so I'm gonna opt for the Vapor 9s and a heel pad of some sort as they seem very comfortable.
Thanks again for the support!