Advice needed - how best to shift a seized hex bolt

[Kittyhawk]

Can't say I've ever had success with penetrating oils on seized bolts, grub screws etc.
My go-to is a butane micro torch which will deliver accurately directed heat directly to the offending bolt. Try several cycles of heating and quenching before attempting to remove by whatever mechanical means you decide on.

 

[Amplidyne]

Just came across this when looking for something else, in a tin of imperial allen keys.
This one must be from 40 years + back, and is probably an "Unbrako" brand.
It's only a small key, probably 1/8". It shows well the abuse these keys will take before snapping.
Sorry about the quality of the shot, but it shows the twist OK.