And so you should be. Your SPA is tax free. In order to obtain your tax code they subtract your annual SPA ( i.e 52 x your weekly pension ) from your standard tax allowance (L) which for year 2017/18 was £11500. Assuming you receive, say, an annual SPA of £6552 then your new, revised tax allowance will be £4948. Your tax code will therefore be shown as 494L.
SPA has never been taxable and this how the HMRC deal with it.
I got notification of my code a month after my first receipt and that is how it was illustrated.
They have made an error and sent you a notification letter based on next year's new personal tax allowance increase by £350 up to £11850 as if you were not in receipt of a SPA. It should have stated your new code had been calculated at 529L, using my example.
Write to them or simply wait and in ten years they'll ask for you to return £ 12,000.
I omitted to mention, just to make things clear, the first £5290 of your other pension income will therefore be tax free.