So.... the 1.005% wear on my chain was rapidly approaching the park 3.2 checker needed some pressure to drop in.... I changed the chain, thoroughly cleaned the EX1 cassette and jockey wheels and installed a new 16t sprocket. ... after removing the "Mud Stop" (bit of a reassuring battle lol).... well.... the inside was dirt free, there was a bit of wear to the black casing finish as I had expected. But a quick degrease of the mud stop and clean of the drive splines and regrease, all put back together and torqued up ( treated myself to some tools I now know are necessary! )..... very impressed at how clean the whole thing was and the grease on the crankshaft is holding up brilliantly too.... verdict? Get one.... prevention is better than cure....