Thanks very much for all that advice aland.
I'd sort of decided to go with the progressively larger drill bits idea as I dread the thought of having an extractor snapping off in there, so your ideas reaffirmed mine.
The anti-clockwise drill bits are a good idea too, so I'll go shopping for those, and I might also get a set of tapping bits or whatever they're called to remove the rest of the bits of bolt thread from the hole. That way I hope to ensure I can use the proper Volvo subframe bolts again in an undamaged hole.
Thanks again for the advice. Nice to know that I wasn't the only one thinking like that
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Aidan