You don't even need any mountain, wave can happen over flatland in some circumstances : "Cloud streets have also been observed to force wind from a right angle up and over, undulating downwind especially when there is another cloud street at some strategic distance between wave crests to enhance the wave shape and intensity downstream." (
here).
Coudn't find a picture online but there is one in "Cross Country Soaring" by Helmut Reichman (p56 in the French edition).This kind of wave do happens from time to time in western Europe and I've been lucky enough to experiment it once (1989).
However, this requires a cloud-street, i.e. a convective layer from ground to the inversion layer and a stable one on top. Given the temperature and QNH, I guess the bottom layer was stable too, so I'd go for the windshear at the inversion layer