I've soaked it in WD40, put an extra long extension bar on the allen key (actually an 3/8 drive Allen socket) but all this does is chew up the metal.
Before there were no flats left I've had an impact driver on it for about 1/2 hour, hitting as hard as I dare. I've tried gentle heat from boiling water - don't want to get blowlamp on it - and I've tried tightening it a bit to break the threads.
My only thought now is a proper impact wrench - but I don't have one

Any other ideas?
Thanks, J