Retrospective taxation

I was listening to Vince Cable today on the radio whilst driving between meetings. He was commenting on situations where rates had been backdated after they had been re-assessed by the Valuations Office. I know that this has been a particular issue in my home town of Grimsby where businesses on the docks have been left with large rates liabilities following a decision to assess the indvidual businesses on the docks and not Associated British Ports (ABP). 

Whilst that's really good news for many businesses my ears pricked up when he said that he didn't support the backdating of tax. That will be interesting in the context of the budget in a few weeks time when we see whether any taxes will be backdated to the start of the fiscal year.