Last time I needed a double sided sticky pad for a rear view mirror, I found Ford were cheaper than Halfords. And worked.
You will need to ensure both mating surfaces are cleaned with a degreaser/solvent, with every trace of glue removed, and In this weather, both screen and mirror are completely dry, which probably means having heated front screen on with A/C running, blasting hot air up the screen! Any condensation, even stuff you cannot see, will probably mean it won't stick very well.
Happy Xmas to you and all our readers!