For reference here is an alternative method (the one I used in the end, although I prefer Fatherjack's solution)
select NINo from NINoTable
where substring(NINo, 1,2) not in
(select chars from NINoReference)
For reference here is an alternative method (the one I used in the end, although I prefer Fatherjack's solution)
select NINo from NINoTable
where substring(NINo, 1,2) not in
(select chars from NINoReference)