Second option, to the shed for pliers, tried twisting the same way as the key, only for the tab to snap off clean with the tin!
Third option, (and i'll admit in anger) used an axe to cut around the tin. made a mess but got it out and didn't cut a chunk out of myself!
So, for future little things that make you scream, has anyone got any ideas for opening a tin of corned beef if it A) is missing the key and B) has a snapped off tab?