Interesting, this is my second exhaust, and fatigue from vibration
seems to be the cause. - I've got about 7000km on mine.
Its definitely vibrating more than I remember it doing, so I will have
to investigate further. Anyway, i'll have a look this weekend. Cant
imagine its mains, as I managed about 250 miles on it before the
exhaust broke, and I would hope that the mains would have let go over
this sort of mileage cruising at 55mph.

The vibes only really start above about 40MPH.