Thanks for your replies folks.
I don't really have much against a windscreen mount but I am fearful that it won't hold. (Old) Renaults dont have a great heating system and I have to wait for ages until the windscreen is dry. Then it vibrates quite a bit during the journey. Maybe I'm just being too cautious.
I phoned Renault for advice but they were, surprise surprise, non-commital and wouldn't say if they knew of any that would fit.
I think that, short of permenantly fixing something to the dashboard, I will have to stick with the windscreen mount.
Ho hum...