As above out at Button Oak on Sunday and when i washed the bike I could see a black resin on the exhaust, turns out its the plastic cover that's melted to it, anyone experienced this before, know what the cure is?

It's often caused by the exhaust gasses blowing around the pop rivers.

It can just be from wear and tear but the main culprit is when the silencer takes a knock or 3 and the holes on the alloy sleeve get stretched and elongated.

Drill out the rivets, clean up, reshape for a good fit if necessary, re seal with a heat proof silicon or exhaust paste and rivet back together. 

Its worth doing sooner rather than later as the rivets make a mess of the alloy if they are left vibrating about

Did mine on 2017 rr during Lorax S3 round .

I am heavy on air box and exhaust covers over the years - must be my grip on tight and go method of riding ??

Think covers are dirt cheap £20 ?

check for leaks as Jr says and that the little rubber bung is in place 

presume it has melted where your boot rubs

I fit heat reflective self adhesive material to the inside of mine, like they use on mx bikes as the 4rt silencer has always run hot.

The 300 plastic cover is the same as standard but has a small amount of heat reflective tape on the inside, as they get hotter still. 

It is where my boot rubs, i don't think the rivets are loose, Ive had it all apart and it felt very solid, rode without the cover Friday, i had a red mark on my leg for 24 hours

Treat yourself to a new cover