Thank you all for those suggestions and advice.
I do think that BMW would have replacement screens for their own hardtop, but my problem is that we have a Weismann hardtop with the bigger glass. I did send an email to Weismann, but as you probally all know, they are under new ownership and have left the past (in terms of parts) behind. They didn't even reply to me :-( . I was hoping they might have even pointed me to their old supplier. Perhaps I will try calling them.
In answer to Pete's question as to what is wrong with the glass screen, it is actually ok and all in one piece. It is the heating element in the glass that is broken, and also i think the male connectors to the element itself are broken off. In these cold mornings the rear view is often misty on the glass, and impossible to see through.
Actually, this leads me to another question and one I believe was addressed on our forum here many years ago...do our Z3's have heated wing mirrors ?
Thanks again guys.
Liam.