"Richard J." wrote in message
. ..

The logic is to use the last two digits of the year for Mar-Aug
registrations, ditto plus 50 for Sep-Dec, and the same code for Jan &
Feb of the following year. So Mar-Aug 2011 will be 11 and Sep 2011 to
Feb 2012 will be 61. This formula will be valid until 28 Feb 2051, the
last two 6-month periods using the codes 50 and 00.

Thanks Richard, Neil, Annabel and others for clearing that up (sorry for the
delay in responding, I've been away over the Christmas/New Year period). I'd
clearly been incorrectly informed about the third/fourth digit structure. As
it was explained to me at the time the third digit would *always* be 0 or 5,
dependant upon month of registration, and the fourth digit would always be
the last digit of the year - hence my confusion! Obviously I was
misinformed.