Thanks Chris and Kev!
David, I did use M8 bolts, they were a bit of a tight fit, but went in with a some persuasion.
The spanner clearance issue did have me swearing a bit...
I ended up packing the nuts out with a handful of thick washers so I could get a socket on them. Had to grind a bit off the washers to get them to fit too
Still, it's better than having a UJ that fails IVA!