Re your tax code change.
It sounds as though you are making the assumption that the new tax code will be operated on a cumulative basis when it will almost certainly be operated on a non cumulative basis.
Resulting in a much smaller deduction than you are expecting the first time the new code is used.
Thanks.
The new taxcode is K391X which replaces 1007T. I did wonder about W1/M1 but my googling suggested that X just means "will be reviewed at EoY, not that it's also a W1/M1 code (I looked here:
https://www.gov.uk/tax-codes/what-your-tax-code-means
and the notes on the back of my coding notice Tax Code ending in X "Shows we will review the tax you have paid after 5 April". I'd have expected M1X or something like that for W1/M1 basis.)
I believe I have been underpaying tax at a rate of about 400 per month based on my EoY projections. So to catch up I'd expect my end September paycheck to be 2800 higher than August's figure for tax deducted. It's not a huge deal for me whether they collect it in one go or in 6 instalments, I've already put the money aside, it's just whether I have to trickle it back or do it in one go (Six equal instalments makes a lot more sense based on what the notice says and means I don't have to worry about ensuring there's extra money in my account by just after payday.)