Question Author
Panic over, I have discovered an error two years ago that has caused this.
I am not liable for this tax, but now have the task of proving this to the tax man.
The employer for my second job changed mid year, and I was transferred across under TUPE regulations.
At the end of the year a mistake was made on the P60, whereby the earnings from the previous employmer were added in twice. A handwritten re-issued P60 was given out, but the taxman has calculated using the original incorrect P60.
I can sleep well now.