Screw this!

[Streetwind]

Update: turns out our electric drill has a cutting disc attachment, and a friend of the family owned an impact driver. He came over this weekend and we managed to cut the slot in the screws. They were still tough as nails, no pun intended, with even the impact driver needing a good number of tries to get them to budge. But ultimately it did its job as advertised, and we got them all

Thanks for the suggestion!

[Seret]

Screwgrab. Amazing stuff. Between that and an impact driver if required you should be able to get any knackered old screw. Having a screwdriver that you can get a spanner onto can help as well.