Thanks to everyone that responded. Unfortunately none of the methods will work, having a rain sensor means that there is a thin circular bead of glue/bonding stuff that secures the mirror to the windscreen and not enough room to use any pad or glue/mesh.
The solution…
Well your not gonna believe this, but I went to the RAC fitting place in Northampton (top blokes), the only real way to permanently fix the mirror back is to get a new windscreen!!!!! A new windscreen comes with a clip that holds the mirror, this is bonded to the windscreen at the factory. Fortunately for me, I had my windscreen replaced last year by the RAC and it had a three year warranty on it, so they are replacing it tomorrow for free!