Off the screen or mirror mount?
Anyway try to scrape the bulk of residue off the surface.
Different adhesives react differently to different solvents.
Try if you have some, electrical degreaser, Brake and clutch cleaner, Isopropyl alcohol, occasionally screen wash concentrate..
If it is all gooey. place an icecube (wrapped in poly bag) when cold will scrape easier.
After all that halfords may have some stuff just for the job.