Evenin' Craig...
That's a bummer
...very sorry to hear that.
Reckon though a likely cause for the
over-tightened fixing bolt may be a Loctite or similar
liberally coated over the threads before it was previously returned...anyway before you attack it with the Black n Decker...and if your able to get hold of a single hex socket (preferably 3/8" drive), I first try a one or two well placed hits directly to the head of the bolt with a hammer and drift punch (rationale: if it has been 'fixed' it, one or two blows ought to help loosen the thread)...then try the single hex socket (rationale: the rounding caused be the double hex should
bite on the flats of the single hex).
And if your not going to replace the copper washers...then if you have a source of heat (like a blow lamp / torch), give them a little heat (discolour), to anneal them...and return the bolt with the annealed washers with only a little oil on the thread and wrist tighten...they wont (cant), go anywhere.
Good Luck with it...and if you really get stuck...send me a personal message...I have a Mate near you that may be able to help you
Cheers the can :-)
Just a thought