Wise words, methinks.
The irony is that, via an indirect route, my immediate problem is solved.
Because I've now bought the Canon 24 -70 f2.8 L lens, the 17-85 kit lens, with its 67mm diameter is no longer is use (might look to sell it - it does seem to still have a value). So ...........
The problem occurred on the 70-200 f4 IS L, which also has a 67mm diameter. Therefore ...................
I've taken the filter off the kit lens and put it onto the 70-200.
Adaptor Ring? I had two of them as well so that I could leave each on the lenses to make fitting the filter holder much quicker.
So problem solved .................. except I still have the other UV and ring stuck together.
But, in anticipation of the problem arising again some time in the future and as a means to try and finally resolve the present predicament, I think I might take Richard's advice.