I lived in Calne for 8 years and drove that way to the M4 about half the time. Always had running water on the first steep downhill left hand coming out of Lyneham (in Winter not unusual to see cars down the bank due to that). Don't recall seeing running water on the part of the road down Dauntsey bank. But it did get more and more undulating as time went on. They re-laid the top of the bank (the section that failed here) at least once in that time.