Not sure if the saloon is same as tourer in the way the caliper is held. Mine is a tourer, and if your's is a tourer, then I know what you mean by the two 13 mm bolts (more correctly, hex-head set-screws)
If it's starting to round off, then try some big mole grips if you have any. Otherwise a pipe wrench might do it, the sort that tightens its grip as you put more leverage onto it.