I need to reattach the rear view mirror to my gf’s car windscreen. What’s the best sticky stuff to use?

I presume double sided sticky stuff of some flavour?

Tried generic b&q pads which had a very short lifespan.

You can get pads or glue from a car part shop.

Or go to the dealer for your brand, they’ll probably have the correct pad.
You could also try a windscreen replacement place.

Locktite are usually the go-to bods for such things:
http://www.halfords.com/motoring/paints-body-repair/adhesives/loctite-rear-view-mirror-adhesive-kit

3M Pads off ebay I find are the best, use them for my in car camera also and they’re great.

I went to the main dealer and got one for very little money.