I think I read this jar-opening tip here on Gransnet within the last couple of years:
Bash the side of the lid hard (I put a cutting board on the floor and hit it against that).
I presume that it distorts the metal lid slightly, thus breaking the seal, and then it will open quite easily.
It has worked every time I'v needed to do it, though just once I had to do it twice, as the first time I must have been too gentle
Thank you to whoever it was who shared this tip, as I haven't had to run out and greet my neighbour with cries of "I'm so pleased to see you- I need a man" since then, and I'm sure he is very relieved