The regristration numbers are split into 2 6month blocks.
The first 6months follows 01,02,03,04,05 etc etc
The second 6 months follows 51,52,53,54,55 etc etc
Basically, the first 6 months follows sequentially, the second 6months has 50 added to it to signify it's the same year block but the second part of it.
Registrations change every 6 months. They used to alternate between a range starting with 0 and one with 5 eg 02 and 52. Those obviously ran out at 09 and 59 and so have swapped for ones beginning with 1 and 6 eg 10 and 60.
the year has 2 parts the last 2 digits represent the year and the part thus
09-59 for 2009
10-60 for 2010
11-61 for 2011
you may have spotted that generally for the first part of the year the 2 digits represent the year and for the second half of the year it represents the year + 50
The first part starts 1st March the second part starts 1st of september so a car registered in January 2011 will have a 60 plate, rather than an 11 plate.