Hi Charley,
Just one observation:-
As mentioned, the PAYE system is cumulative. Therefore, your tax credits cumulate each month, i.e.,
Month 1 443.08
Month 2 443.08 x 2 = 886.16, etc.
Your tax credits are correct for January, February, April and May. However, have a look at your March tax credit. It should be 443.08 x 3 = 1,329.24 rather than 1,280.51.
This could be down to a payroll error, which should have rectified itself in April, when you were given the correct cumulative tax credit.
How much tax did you pay in January, February and March?