After seriously considering d), I used an LPG blow torch to heat up a 3/4" piece of bar until it was just below red hot. I then used multi grips, to hold the end of the bar onto the heads of the bolts for about 30 seconds each.
After letting the bolts cool properly, I tried undoing them using a heftier ratchet. I got both to move a little, so here's hoping they won't break when I get a new O ring and try to replace it.
( I replaced the bloody O ring only a few months ago, and I still thought it needed a new gasket. Maybe too much of answer d) in my youth).
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