Edge 840 Gradient issue

Recently purchased (2 months ago). Just this week it started showing completely wrong gradient on my rides - Negative when going uphill, positive downhill, climb pro- showed a climb but the gradients within it suggested I was going downhill. 

I was advised by support it may be the barometric altimeter and to follow steps including running it under the tap. Support has now gone quiet on me! 

Anyone come across this and has an easy fix?