In reply to brunoschull:
Thanks for the huge reply
I'll try to work my way through it;
This should be a relatively straightforward problem.
"the dropouts on the bicycle are a little worn, the locknuts on the hub are a little worn, and the quick release is old, or simply not effective"
The QR might not be that good, certainly, there's little to no wear.
"If you need to use the bolt on hanger, so be it, but you certainly don't need any additional washers"
I need at least one additional washer as the bolt on hanger catches on the cassette lock ring otherwise. I put one on the other side to even things out, maybe I don't need to?
"One of the first steps I would recommend would be to try a new quick release skewer, well-lubed, and closed tightly, but not with excessive force. It should leave an imprint in your palm, and require a strong push to close, but you should not have to really strain to close it."
"First, some basic wheel work. With everything in place, grasp the wheel with one hand, and move it back and forth in the frame. Is there any play or rattling from the hub? Now take the wheel off, spin the axle, move the axle back and forth, and so on. Is there any play at the axle? Do the bearings run freely with no binding? If there is any play or the bearings are too tight, get the hub adjusted properly. Next, put the adjusted hub back on the bicycle and spin the wheel. Is the rim true or does it wobble back and forth? If the wheel does not run straight, get it trued. These two things, the hub adjustment and the rim truing, are not directly related to your problem, but since you're working on your wheel, why not do it right?"
The wheel is pretty much as new.
"Now check the following things: measure the width of the axle, from locknut to locknut (the parts that contact the frame). Second, measure the dropout spacing on the frame. Make sure the hub and the frame are the same spacing...you don't want to try to force an narrow hub into a frame with wider dropouts."
The drop outs are the same.
"Third, get the dropouts on your frame inspected and aligned if necessary."
This could be a good idea. Any ideas where I could get this done - I live in the North East.
"OK..assuming all that is fine, on to the part that is probably the most important. Inspect the surface of the locknuts on the hub. They usually have ridged or raised surfaces to bite onto the frame...are they worn down? Do they need to be replaced? Do the same with the quick release skewer: are the inside surfaces worn down, or do they look good?"
Looks good to me.
"If none of the above works, you could convert you hub to a solid axle with fixing nuts, which generally generate much more clamping force than quick releases. To do this, you would need a solid axle of the right length, fixing nuts, and so on, but it might not be so simple, because the threads on the new axle might not match the threads of your current axle, so you would run into compatibility problems with the cones...and so on.
In terms of how to get all this done...you need to find a good bicycle shop with mechanics capable and willing to perform this kind of service. At one time, things like aligning dropouts, replacing axles, and so on, were routine, but, as they say, "the times they are a' changing..." Nonetheless, a good bicycle shop with some older experienced staff should be able to help you with all of this."
Yeah that's the reason I'm on here - the three bike shops local to me mainly just sell new posh bikes and didn't really know anything about it :-P
I might try changing the lock nuts see if that helps. I'm beginning to think that the problem might be down to the drive side washer allowing that side to slip. Is it possible to get a lower profile lock ring for a 105 cassette? It needs to be about 1/2 a mm thinner.