All major load bearing bolts on Vauxhalls are hex head bolts, not a torx headed bolt like that in the picture.
And by load bearing I mean engine mounts, gearbox mounts, engine to gearbox bolts, strut to hub bolts, wishbone bolts, subframe bolts.
Torx bolts of that length are generally used on things like alternators, air con pumps, PAS pumps, cam belt tensioners/rollers, petrol tank straps etc.
I can't picture that particular bolt anywhere on the back end of a Corsa B though, its definitely not part of the exhaust system (and I'm 99% sure its not part of the rear suspension set up).
With that point on the screw and the rubber washer I would say it is a tension screw for a belt. Might be from the belt tensioner for the water pump/aux belt.
This would also explain why only some of the thread is dirty/worn at the end.
I think it is a tensioning screw and in which case, you should get it checked pronto.