Ridinmurray, I have exactly the same problem and my solution so far is as follows. Do the usual things such as hydrate appropriately; take electrolyte supplements, and move your cleats as far back as possible. If no cleats, put the pedal spindle near the arch or your foot. Then check to see if your saddle might not be a bit too high. Finally, pay close attention to everything about your position on the bike.
My last ride yesterday was 60 miles and the calves were seriously threatening to cramp but didn't quite. Over time, they are getting very much stronger and able to deal with the load imposed by cycling.