Trust me on this, I am undoubtedly being unbelievably thick. I'm generally pretty good as a mechanic but at a glance it looked like that cover had to come off to get access to the sprocket cover. I'm at work at the moment but, if I get a chance before I go on holiday tomorrow, I'll have another look. I'm prepared to be laughed at and generally made fun of if it turns out to be simpler than it appears.