Went to poundland and bought 2 tubes of locktite gell superglue.
I took the mirror out of the housing again by moving the mirror from controls by going maximum down and maximum left position then putting my fingers in gap you creating on the top right and gently pulling, working way round the new gaps created, there are 4 tabs. Then disconnected the two wires located on the top left corner, they are miniature spade connectors which heat the mirror.
The pad that holds the mirror in place was only holding it about 30% accross the top of the mirror. I was initially thinking to just put some glue on the part of the mirror that had come away from the back plate but now decided to completely take it off and do a proper job. I gently prised off the the sticky part to completely separate the the glass from the back plate.
Upon inspection I could see that it's the back plate that is heated and then via conduction the heat travels from the back plate to the glass.
I dabbed the glue evenly onto the back of the glass being carefull not to put too much near the edge as when pressed it will spread and go over the side. Then carefully aligned and mated the two surfaces together.
Its says on the locktite gell packet that it has nearly instant hold but I applied gentle pressure for about 30 seconds, did a wiggle test to check adhesion and it was holding so went around the edges pressing them together and then the middle to ensure good contact.
Let it dry for a few minutes and then reconnect the two wires, make sure you push them in all the way until you hear a click, I nearly didn't and realised as I double checked the connections and realised they should be pushed in further. To be honest I didnt note which wire was on top and which below but thinking back I dont think it matters as it's just a flow of electrons to create a circuit and heat the back plate.
Then centralised the the bit the assembly clicks into using controls to make it easier to refit and pushed it back into place.
Wipe off finger marks from glass and job done.
Took it on the motorway for just one junction to see if it still wobbled and it was perfect.
Took me no more than 10 mins in the lidl carpark.
Hope this helps people, If anyone has any questions then feel free to ask.
Only thing I'm thinking is how well will the glass heat up now and will the heating have an effect on the stability of the glue.