Why do the rich a greater benefit? There must have been some reason, in the beginning, surely?

Just like working farmers whose family have farmed for years. It is understandable that they could have an asset value for their land that would put them into having to pay inheritance tax, and no cash to pay the tax. That's why they were exempt. Along comes some clever tax consultant and Dyson buys a farm which takes whatever he paid for it out of his inheritance tax liability. That's abuse of the system, in my book.

Maybe the Defined Contribution system of pension tax relief is being similarly abused by the rich? Pile money into your pension scheme and you take it out of the inheritance tax system and you don't pay income tax until you or your beneficiary takes money out.