OK, to complete the picture, I am now able to answer my own question and help readers to avoid falling in to the same trap.
- The spring lock washers as descibed in the original question were in fact a "red herring". They actually secure the hub in place and not the wheel.
- The reason why the wheel rim remained tight on the hub after removal of all five wheel nuts was because of the corrosive reaction described by "bg" above. I was aware this could happen, but would never have believed it could result in such a stubborn seizure!
- The solution? - assertive action by an experienced tyre fitter, i.e. a good hard kick in the right place and right direction. Problem solved.
- Preventive measures for the future would be to remove all wheels on a regualr basis and keep the corrosion under control by wire brushing.
- Are alloy wheels worth all this trouble?
Terry Allman (originator of the question)
February 2006