Anyone else...

You have basically admitted that the cause of the problem was when you were dirt tracking, so why put the problem on Triumph(not saying you are going to) as this is not a problem that has ever never surfaced before plus Sheffield steel does not crack have it welded and all will be well.
 

What are you???
The shoulder angel???
 

The dealer i bought the bike from shut up shop last year and the only other dealer in the state hasn't seen this before either. Common opinion is that it is highly UNLIKELY that the muffler hitting the ground would cause the break. Theres just not enough room to flex before the muffler hits the final drive.

In any case a mate will tig it for me while i drink beer and tell him what a top bloke he is