Out of curiosity though, I'm unclear on the difference between cumulative and non-cumulative codes
Non cumulative means that if you're paid weekly, you get 1/52 of the annual allowance (for each tax band) per week, without regard to previous weeks. (If monthly, then 1/12 per month).
Cumulative means that you get the entire allowance for the year to date, minus any pay to date.
So if someone earns as follows - assuming Monthly payroll, with a tax code of 747L (emergency code, cumulative) verses 747L/M1 (i.e. emergency code, non cumulative)
Let's say you earn gross, as follows:
April 0
May 0
June 1000
July 1000
August 500
September 2000
October 250
November 10000
December 500
Jan 0
Feb 0
Mar 100
Then on a 747L code, the tax will be as follows:
April 0
May 0
June 0
July 0
August 0
Sept 152
Oct (74.60) refund
Nov 1875
Dec (24.60) refund
Jan 0
Feb 0
Mar (354) refund
Total £1574
Whereas, on a non cumulative code, the tax will be as follows:
Apr 0
May 0
Jun 75.20
Jul 75.20
Aug 0
Sept 275.20
Oct 0
Nov 3167.06
Dec 0
Jan 0
Feb 0
Mar 0
TOTAL = £3592.66
In all cases, National Insurance contributions are non-cumulative, unless you are a company director, in which case they are always cumulative.
So, if not a Director, NI contibutions will be:
June 47.76
July 47.76
Sept 167.76
Nov 481.76
Total 745.04
and if a director:
Nov 903
Dec 60
Mar 12
Total: £975