MastoDIYers, best suggestion for a solvent that'll get this stuff off a UPVC window frame without damaging the window frame? Apparently it's adhesive from an air conditioner that used to be in the room

I used to repair electronic test equipment and audio gear back in the 60s and 70s and had to clean off lots of stickers. Eucalyptus oil is hands down the best thing for removing any kind of dried on sticky or adhesive residue, without harming the substrate (the thing it was stuck to). If you're not in Australia, you can usually find it at your local chemist (pharmacy, drug store, whatever). Isopropyl alcohol isn't nearly as efficient at dissolving the gunk and will usually require lots of scraping, with a greatly increased chance of damaging the substrate.