Thanks Neil,
The problem is I actually use the area where you have the mirror mounted when climbing, my foam grip goes much further round the bar than the tape in your photo, meaning I can get a nice full hand grip leaning far forward, can't do that with that set up.

I think I'll end up with a mirror fixed centrally on the bars pointing downwards or a "clip on" type that will be a compromise, maybe like this-

http://www.zefal.com/zefal/produit.php?key=472