Square taper BB seized into an alu frame, tried with all my might to shift it.
I know there's a can of Plusgas in my Mother-in-Law's garage…is it worth nicking it?
What then? Pour it down the seat tube and wait a bit?

I used it on a seized BB, and had some good success. I poured it down the seatube, but had a tray underneath to catch it as it drained through. Repeated a few times and left to soak for a while (some weeks – it isn't a rush job).

i had a siezed in BB once, i was trying to undo it the wrong way 😳

I've never not got a BB out without the application of enough leverage.

I've just freed a very stuck square taper BB from a steel frame. Check you know which direction to undo it first, check again and then check once more for good measure. Put a mark on the frame and BB so you can see any small movement. It's also worth bolting the extraction tool to the axle to stop it slipping. I ended up heating the BB shell with a blow torch and hitting a 2' spanner with a copper lump hammer to finally free it! Yes I will have to re-paint the frame but it was already slightly rusty.

I'm changing a BB for the first time tonight, you guys are giving me nightmares!!

Put the tool into a vice on a workbench. Then use your frame as the lever. Should free up then.