Last week the British government announced changes in annual tax rates for private cars. The rates vary from zero for low emission cars to £215 p.a. for a high emission car (an SUV?) using diesel. There seems to be no car on the market which qualifies for the zero tax (less than 100g/km CO2) so the Prius is now the lowest taxed car on British roads at £40 p.a. Last year I paid £65 tax on my Prius, and £165 on my VW Passat. You can see the details on www.vcacarfueldata.org.uk (Click on VED calculator or click on Database to search for a particular model)
The official site does not seem to make it clear whether Prius is regarded as an "alternative fuel car" at £30 or "petrol" at £40 in Band B.
I've seen a lot of people confused over this, and really can't see why. Your Prius has always been classed as "alternative fuel" (which also includes alternative technology), and that's why your current tax disc cost you £65 rather than £75.