can anyone recommend the best way to decoke a 2006 125 sherco exhaust

Depends how hard it is.

If its oily fill the exhaust with cellulose thinners and it will dissolve.

If its harder you will need somethig like caustic soda or methylene chloride.

For any of the 3 you wil need to make a bung for one end and leaved the exhaust ful at least overnight then give it a good flushing out.

Cheers

Don't get use the caustic soda if it's the main muffler section you're trying to do, being aluminium you'll end up with perforated tubes & packing sitting on floor but not much else

There are a few threads here describing how to clean out the exhaust including burning it out