Got the bitch. Took another 2 hours and a visit to the local engineering shop. Got the guys to chop the end off a sacrificial 8mm 12 point spanner (it's an external Torx (six point) bolt - 6x2=12) and chop up a piece of steel tube and crimp the end for a tight fit. It was my last idea really. Only had about 1 degree of rotation to crack it but after a few tries I heard that reassuring break. Still took another 10 mins to wind it out, used a tiny ratchet to do that 1 click at a time...