Soo, had a go at replacing the front struts. Usually a 7mm allen key and socket are enough to undo the suspension top nut until...

When trying to undo the nut heard it crack and thought I'd got it moving, until I realised I've now rounded the hex part of the strut. Any suggestions on what to try? Seen as the top of this strut is now buggered I'm resigned to the fact of just getting the angle grinder and cutting the top of the strut piston off.