I seem to recall the monthly cost will equate to the same over all cost as buying 6 months at a time, which I seem to think is being dropped from 10% to 5% over paying annually.
There is, of course, a fair chance that this complete b*ll*cks that my brain has just made up to make me look stupid for sh*ts and giggles. It does that from time to time.
ETA:
Yay, my brain doesn't hate me today ;
http://www.hmrc.gov.uk/tiin/vehicle-excise-dd.pdf