Hi Ian,
My solution is just a one person spreadsheet that calculates the NI due on NIAble pay. All you do is enter the amount the NIAble pay in one cell, the new NIAble pay value and the differeces are given on he spredsheet in the same manner as the NI element expects, i.e. Pay Value, LEL, UEL etc.
Its not as magic as your version matey :-) Its more for use in a payroll department when you have a small number of NI adjustments to make (in year).
I find that most Payroll departments have no idea how to calculate PAYE and NI manually hense my spreadsheets to assist.
Cheers
Paul