I just saw a thread on this problem, apparently an M16 bolt and 2 nuts can get it off, I just need to find a builders merchant who sells M16 bolts! Like bloody rocking horse shit.
A good quality adjustable may work though. Mine is made from metallic butter it would seem.
Daniel Lewis & Son, top of Hackney Rd sell everything. They'll be able to sort you out.
http://maps.google.co.uk/maps?q=hackney+rd&ie=UTF8&hq=&hnear=Hackney+Rd,+London+E2,+United+Kingdom&gl=uk&ei=b30GTNjdNMah4Qb4q-2hDA&ved=0CBYQ8gEwAA&ll=51.532394,-0.058456&spn=0.017353,0.032315&z=15&layer=c&cbll=51.53237,-0.058605&panoid=eMaPC-niHd8Ugmk9GUS8MA&cbp=12,342.43,,0,6.91
I'll bring the Park along too just in case.