Add numbers missing from 1-30 in a list

[lukestkd]

Hello all,
I have a list of numbers in range A2:A33 that correlate to a position on a seating plan. For some groups there may only be say 15 students, but I need to make sure that the numbers between 1-30 not used are in the list.

e.g. if I have 1,2,3,4,5,6,7,8,9,19,20,21,22,23,24 then the code would put 10,11,12,13,14,15,16,17,18,25,26,27,28,29,30 (each on a new row) in the range A2:A33 below the other numbers.

I have attached a sheet with an example of what I need. Ideally I need it to operate on the current sheet as I'll add in a button on each sheet.

Thanks in advance

Luke