The problem with the windscreen docking unit, (as they call it), seems to be a very common one. Many users are reporting problems on the Tomtom Go forum.
There is talk of TomTom releasing a modified mount in about a month. The windscreen docking unit has to be used in the new range as, unlike my Go 700, power cannot be connected directly to the unit; it has to be supplied via the mount.
This is the reason why I decided against buying the 910. I have contacted the CEO of TomTom asking this to be borne in mind when new models are designed. He thanked me for my feed back.
I await the release of the x20 series, which I guess will be next spring or summer.