I think your start position gives you a philosophical difficulty.
You have mentioned 'need' as dividing line and here you accept that entrepreneurs are essential and should be rewarded for risk taking but should, in effect, stop when their wealth exceeds a certain level. That's a bit like saying Lewis Hamilton should drive more slowly to avoid unfairness.
I think that, in a capitalist system, you are bound to have super rich people and we should tackle it thru taxation, rather than castigating them for being unethical. Yes, I know they will duck and dodge and practice tax arbitrage, but it should not be beyond the wit of man to extract some more from them.
How about an annual audit and a tax on world wide wealth? After all, if I have £500bn assets, I wont miss 1% of that.