Could you not drill it out and then re-tap it?
Dont use an electric or battery drill, you would need to use a hand drill, and drill slowly keeping the bit cool.
Unless you can afford an intrinsicly safe drill, no sparks!!!
Is there enough showing of the head to cut a new slot for a screwdriver? again use a junior hacksaw and cut slow to keep the heat down.
If in doubt make sure the tank is empty then fill to the brim with water, it is the vapour which is most dangerous and highly explosive.
Personally i would go and get another tank, i dont think i would attempt it. If the tank is off you could try a local garage to remove the offending bolt, they would have the expertise to do it safely.
A half empty tank is a bomb waiting for some form of ignition to use as a detenator, a small spark is all it needs, and you wouldnt have to worry about getting the bolt out then.
If it was Diesel it would be less hazardous, I really cant emphasise enough how dangerous this could be and advise going down the professional garage route.
Hope this helps.
Where should we send the flowers :-#
