Re: Hardly rocket science
Nooo, I think not. The partial months kept will only be a single month of the duty (1/12th) in profit for each car sale (from the start to the sale date for the buyer, and from the sale date to the end for the seller), so if total yearly was 6bn and a quarter of cars were traded that's about £125m in this "double take" duty.
Plus they have to pay the people to process the transfer for ownership, etc. Since it costs nothing to change ownership of the vehicle itself, the admin costs in processing V5's (or whatever they're called) has to be taken from somewhere... You could call the extra month that they take a "stamp duty" on car sales to pay for that admin.
But despite this I still think the whole duty calculation and collection isn't right. I'd rather see an increase on fuel and road tolls than have a set price to pay irrespective of actual use.