Hi Pete,
If I were you I'd get along to a physio asap, before it becomes chronic or - even worse - you tear something. You may have a biomechanical problem that orthotics would help to clear up, and/or it may be that you'd benefit from some specialised stretches, or even just from a particular sort of shoe. A physio with running expertise is in the best position to advise you. I developed an achilles problem and it's just beginning to clear up over the last month, since I began to use custom orthotics. Best of luck :)