For the benefit of those who have the same problem I did, with the benefit of hindsight,
1. Remove small cover over exit hole in muffler
2. Locate the whole unit so the hole is facing down - eg on a small workmate bench so you can access the hole from underneath (this is important as otherwise any loosened rubbish could access the exhaust port and damage piston/bore
3.Push a long skinny screwdriver up there as far as it will go and move it around ensuring any loosened rubbish falls out.
Estimated time to completion - under 5 minutes
(Time spent learning this is the easiest way - 2 hours)
