Get can of expanding foam
Find extension for plastic nozzle
Liberally lubricate inside of exhaust pipe with soap
Throw piece of string down exhaust past bolt
Squirt foam in bolt area
Have cuppa tea while foam sets
Pull string
Dig bolt out

Find foam is stuck inside exhaust pipe, cut exhaust pipe before foam - you got yourself a nice free flowing, really noisy exhaust.
Get stud out of foam.

Nah, forget all that, too much hassle take the exhaust off m8.

Good luck.