That photo made my life a lot easier, thanks! The tips regarding how to reach down to it (chain a few ratchet extensions together) and how not to lose the losened nut (double sided sticky tape) were also invaluable.
Within 24 hours of discovering the bonding strap had corroded away I had it replaced with a 40mm by 4mm stainless steel strap. Why stainless steel? Simply because that was all that was locally quickly available on my last day on the boat this year, and it should last beyond my lifetime.