Thanks a lot guys, I think I know what the problem is...see the pic...
I had problems getting the optics to work properly (in Windows the cursor would move back and forth while moving the wheel in one direction), so I removed the washer circled and replaced with a thinner one, so my optic wheel is nearer to the optic part circled - this seemed to cure the problem. However this means the optic wheel is also very close to the large gear - I didn't see this as a problem but after a few weeks use I was shocked to see last night that the optic wheel's holes were *full* of grease! I don't know why it even still worked!
NoOne: I guess I did have too much grease on there anyway, so it is probably not a problem. I seem to have put everything else together correctly.
Doc: I wonder why my optics don't work well without having the wheel close to the opposite site of the optics to you? I do have the optics mounted the same way around. Also, does your wheels' shaft have any slack in the bronze bearings? Mine seems to have a bit of 'in/out' travel (probably just a bit of wear) but also some side to side 'give'. Not too concerning, but it just clonks a bit in heated gameplay sometimes...