This year's tax code will never give tax relief for a pension payment made in an earlier tax year.

The relief due for last year will be calculated through your self assessment return and that seems to have happened as expected.

Note as a higher rate payer you do not receive an extra 20%. The pension contribution simply increases the amount of basic rate tax you can pay, which in turn reduces the 40% tax payable. This can be about 20% if you pay enough 40% tax but it isn't a fixed 20% extra just because you have paid some higher rate tax.

HMRC have now included an adjustment to your current tax code on the assumption that you pay the same pension amount again this year. The tax code is allowing relief for the assumed payment in this tax year not last year's actual payment.

In a simple scenario a gross payment of £10,500 will save you £2,100 in higher rate tax (£10,500 x 40% less 20% given at source).

A positive adjustment to your tax code of £5,251 will mean your employer will deduct £2100.40 less tax (£5,251 x 40%).

You can opt out leave the code as it is and claim the extra tax relief due for this tax year on your next self assessment return or you could ask for the coding amount to be amended to reflect the increased payment you are going to make in this tax year.

If you do advise HMRC of this it is important you make it clear what the gross figure is. £14,100 will not generate extra tax relief of £3,525 so I presume this is the net contribution but that isn't totally clear from your post.

Thank you so much ! Very enlightening.

If you delve deep enough into your Personal Tax Account there is a way to update the pension contribution amount on there so that it gets picked up and your code changed. It's well hidden and I can't remember how to get to it but it is (or at least was) there a while ago.

Alternatively a call to them should be enough to get the adjustment made.

Thanks very much!