For a while now, my trusty MX518 mouse has had a bad clicky on the left side (stuck etc...) and was old. I was having to think about buying a new mouse, but! I found a few videos on youtube on how to dismantle them... so i did! I cleaned the whole thing, and it's as good as new! Kind of... but anyway it saved me what? £10,20,30 in buying a new mouse! It was a real mess as it's over 3 years old, with eating at my desk or smoking.
So these vids might be handy to someone with the same problem!
https://www.youtube.com/watch?v=ZDHKqewO4GE
https://www.youtube.com/watch?v=hCkDOAxf5MA