Hello Jez,
I have disassembled plenty of dead mice (mouse's) and found them a SOB to remove the parts. Most of the optical wheels are one piece of plastic which makes it harder to use in a single project. Once you get it out you have to saw off the other wheel which is difficult if you don't how to do that. I found a dremal drill with a grinding wheel does a good job. Once you reach that stage then a whole new set of problems develop. Hooking the optical wheel up to a motor to make use of it. I found that to be a tough problem to solve. But threw it all I have to say I learned a lot and finally did get some use out of something that was going to be thrown away.
Regards,
Charles Bannister