The first 2 letters refer to the year of birth. They started at ZZ and worked backwards. They will end up with AA.
The next 6 numbers, in pairs of 2 are a sequential number. They would start the series with 00 00 00 and end with 99 99 99.
The last letter is either A, B, C. or D. This refers to the day any payment was made, Monday, Tuesday, Wednesday and Thursday. They did not pay on Friday, Saturday, or Sunday.
So for all the Zs there would be 999,999 x 26 giving 25,999,974 people! So any late comers (those born overseas and returning to the UK) could easily have been accommodated within the lettering framework.
Regards
Chas