JohnD said:
the idea is "equal pay for work of equal value"
I'd say that you've hit the nail on the head there. The problem of course lies in how you rate the value of any particular kind of work.
Here in the NHS, our lords and masters actually came up with a solution. It was called Agenda for Change and it involved spending huge amounts of taxpayers' money on a scheme to rate the value of every job in the NHS. Since we're the biggest employer in Europe, that's a lot of different jobs!
First, a large number of existing jobs were given scores in a multitude of categories as diverse as communications skills and the need to work in unpleasant conditions. The scoring method was then tested on the actual employees to see how well their scores correlated with their existing pay.
Next, the scoring system was adjusted for best fit. This resulted in greatest weight being attached to the two categories of responsibility and required knowledge and skills. No surprize there really.
After that, all of our jobs were scored and we were all 'banded', as it is now known. Some gained and some lost but, on average, it worked.
Now look back a bit and see what has actually happened. The scoring system was based on existing jobs, that is jobs whose pay had been determined, as usual, by supply and demand. The relative values of all NHS jobs are now rated according to the supply and demand that existed in 2004.