If the tap wasn't tightened by an over enthusiastic strongman, you might get lucky with a pair of pliers inserted into the void, then opened against the inside surface.
Sadly it appears plumber was doing his best Geoff Capes impression that day.
@TTM started
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If the tap wasn't tightened by an over enthusiastic strongman, you might get lucky with a pair of pliers inserted into the void, then opened against the inside surface.