Thhaankks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
On this current code of K784 this will require an additional £654 of income to be added onto your gross salary (less pension contribution) so a total income of £5379 per month of which the first £2605 at 20% = £521.00 and the remaining £ 2774 at 40% = £1109.60 so total tax of £1630.60 which leaves you £3095.10 take home pay.
( so £41.40 less a month than you take home now )
Let me know if you wish me to expand on this information provided
Thanks for your response
Not as this stage as the computer has to automatically calculate the potential underpayment just because the tax code has decreased, so only appeal this when the tax codes for the following year come out (IF this is then carried forward for collection in error)