Well, I still have my G502 despite the LMB not working at all on it, so I decided to give it a try. The first brick wall I hit, the mouse feet came apart when removing them. Found a new set on the Logitech website for £2. Sorted. :edit: Never mind, they want to charge £10 for delivery!!!
Sadly, the second brick wall I hit was that I don't have a phillips screwdriver small enough to remove the screws.
I'll get one from my local hardware store next time I'm out and see if I can fix this mouse. Cheers!