Are my sums right? Is it worth trying to mitigate?

Albermarle

You should be able to make separate contributions to your workplace pension, but you would need to check with them.
Currently they only receive one employer contribution a month ( that includes your salary sacrifice).
If you add a lump sum personally from after tax pay, then normally a provider adds basic rate tax relief to it .
It maybe the provider can only handle the regular employer contribution, so you would need to check with them. Most will be OK with a one off personal contribution, but some may not.
If it is the latter case you will need to start a new personal pension/SIPP for the lump sum ( easier than it sounds) 

PieHard

I will investigate what is possible for this year and then up the salary sacrifice for 24/25. 

On the actual logistics of how it works, assuming I'm at £154k and want to get to £100k... would I put into my pension £24,300 of net pay, the pension provider would uplift by 20% and I'd then somehow 'claim back' another 25% so that I was only being taxed on £100k?

Albermarle

PieHard said:

I will investigate what is possible for this year and then up the salary sacrifice for 24/25. 

On the actual logistics of how it works, assuming I'm at £154k and want to get to £100k... would I put into my pension £24,300 of net pay, the pension provider would uplift by 20% and I'd then somehow 'claim back' another 25% so that I was only being taxed on £100k?

The pension provider uplifts by 25% ( You need to earn £125 to get £100 after 20% tax ) 
Normally then you inform HMRC of your gross pension contribution ( so including the 25%) and ignoring any workplace salsac contributions.

Dazed_and_C0nfused

PieHard said:

I will investigate what is possible for this year and then up the salary sacrifice for 24/25. 

On the actual logistics of how it works, assuming I'm at £154k and want to get to £100k... would I put into my pension £24,300 of net pay, the pension provider would uplift by 20% and I'd then somehow 'claim back' another 25% so that I was only being taxed on £100k?

You cannot do that by making RAS pension contributions.

RAS contributions to a SIPP or personal pension can change the rate of tax but they do not reduce your taxable income so you would still have the same £154k taxable income, just more might be taxed at 20% and less at 40%/45%.

And if you reduced your adjusted net income (which RAS contributions do) you might get to keep some or all of your Personal Allowance meaning some income isn't taxed at all.

[Deleted User]

PieHard said:

I will investigate what is possible for this year and then up the salary sacrifice for 24/25. 

On the actual logistics of how it works, assuming I'm at £154k and want to get to £100k... would I put into my pension £24,300 of net pay, the pension provider would uplift by 20% and I'd then somehow 'claim back' another 25% so that I was only being taxed on £100k?

In this example your contribution would be grossed up to £30375 (£24300 /0.8). Your adjusted net income would be reduced to £123625. £23625 of this would have an effective rate of 60%.