I've got the stud fixed
BMW told me it needed a new chassis rail and floor pan
A few independants felt that they could do something for a couple of hundred pounds. Eventually I managed to find a local company that specialises in car and large vehicle restoration. They took one look and said it was only a couple of hours work.
Today they succesfully managed to remove the old stud and replaced it with a new bolt. It took an hour and a half and only cost £40.
The towbar is now fitted and I've only got the electrics left to do.