Interesting picture but certainly only a small part of this bike can be M20. The front end is not. Both are sitting on frame as the rear wheel/mudguard are free of passenger. Elongated frame? And there is something shiny under the tank, where the head would be expected to be, which appears to be some sort of OHV gear or covering for it.
The bike is a military M20 but it had been converted with front and rear suspension of a Horex. The man in the photo had a motorcycle shop there and had loads of parts. Here an other photo of some of his stock.
That pretty greek girl riding pillion is my wife Linda
I thought that the orange mudguard/wheel may be another bike but upon enlarging the image it turns out that there is no visible means of support of that "other" bike: no rear stand exists and the bike is too vertical to be resting on a field stand or a kickstand. Henk: any clues?
P.S.: I didn't realize that your wife is Greek. No wonder you visit Crete so often.