I also agree with you that it's important to get an accurate read on how much an employee earned. My point is that there are some people that have an incentive to obfuscate this amount and that I can only see one unambiguous way to avoid this obfuscation.
I think the starting point in these discussions should be what someone was actually paid, not what they might have been paid. Then if that amount can be justified by unsociable hours, extra hours, or whatever, then that's fine. But if the various hours, loadings or whatever seem unreasonable then that's a different discussion.
I wouldn't even normally be interested in what someone's paid - it's not my business. However, the car manufacturing industry is a different case since there was an expectation that tax payer's money should be used to bail it out. In my opinion, tax payers deserve to know what it is that they are paying for, in detail. Particularly in the car industries case since it seemed like every time that the government tipped in more money that it was closely followed by the workers getting a pay rise.