DIRT tax and budgetry increase

I had a one year fixed rate account with Anglo, it matured earlier this month, upon further investigation I noted that DIRT was calculated at 27% backdated to date of account opening in Feb 2010, I queried this and got nowhere.

My understanding was that DIRT when amended in any budget took effect from a date shortly after the budget maybe even the next day, in this instance the ' old ' rate of 25% applying up to that change date.

I'm feeling like I've had some of my hard earned pinched right out of my pocket with this additional 2& DIRT backdated..., any wisdom appreciated...?

The increased rate came into force on 1st Jan.
The rate applied is the one on the date the interest is paid, not when it was accrued.

As far as I know ( and I could be wrong) DIRT is deducted at the rate applicable on the date the interest is paid.