I disagree, I do see progressive tax as a counter-balance to lopsided salary calculations between the top and the bottom.
Does anyone, by any measure really think that Bob Diamond for example is worth 1,000 times the wage of one of his cashiers? Don't get me wrong, I agree with the principle of capitalism, those that work the hardest/have the best ideas/skills get more money than those that don't.
I don't know about Bob Diamond, but I could definitely see how person A can be worth many times the value of person B to a company, depending on the impact they have.
Having said, I don't think it always works because those at the top have a lot more power to influence pay rises in their favour and I believe they use that to demand wages that both cover their increased tax give and provide them with the disposable income they are used to. The 50% rate for example, I believe the rich just paid themselves more/demanded more to cover the increase.
More likely they just didn't pay it by shifting it elsewhere, because very few people at the top of companies get most of their income via salary, which Labour knew when they brought it in, it wasn't a serious attempt at anything other than laying a trap for the next government.
Hence why I would introduce personal tax codes, every person's would be unique to them and your income tax would be based on your employers generosity or greed. So if you work for a company that pays minimum wage whilst lavishing it's bosses with multi-million pound packages and you are at the bottom you'd pay no tax, your bosses would pay 70% However if your company pays say the boss no more than 50 times the average salary of his lowest level workers they would all get a flat rate.
That sounds like an utterly terrible idea destined to be both massively bureaucratic and horrifically abused by future governments to better one group or another.
Wages are defined by what a company values your contribution at and how hard you would be to replace, interfering with that rarely achieves the desired aim.