Huge amount of non-coded Tax appeared in my account...

I just received a letter to say my tax code has changed to 742T - looking at my account on the gateway, I can see a 'non coded' amount of extra income that's effectively the same as my PAYE salary. I don't have any extra income, so have no idea where this has come from. As a result HMRC think I earn twice as much as I do. (I wish!) How has this happened? Why on Earth have they got the impression I've earning tens of thousands of extra income?!?

Hi Martin Nike,
You will need to contact HMRC to review/update your tax code:
Income Tax: general enquiries
Thank you. 
 

It’s because they (HMRC) are not fit for purpose. My wife and I have so many issues it’s beyond unbelievable. Get them solved? Nope.