Sam, given the time scale, go to a sports physio if you can, to get it really worked on. Otherwise, if you have a partner or friend who is willing to massage it for you, get them to do so (starting gently, then firmer, and press-and-hold into the knotted area). Third best is to massage it yourself (baby oil works well for lubricating for massage; Boots own brand doesn't smell too horrible). Alternating heat and ice might help (seemed to help me after a cramp some months ago). Consider taking some ibuprofen to settle any inflammation. A few weeks ago I had a calf that was starting to cramp fror a period of about two days and it settled after three sessions of massage (I'm lucky my husband was willing to take the time to do this) over a period of 24 hours.
Once it's settled, make sure you stretch your calves very well (gastrocnemius and soleus) after EVERY run and massage your calves night and morning. Consider wearing compression socks or calf guards when running - I know sevel people who swear by that.
Good luck!