Earnings Letter - not collecting PAYE!?

[nafe12]

I did some work where I was only paid a gross wage (one off total of £1500).
The idea was that I was paid the full amount, and left to deal with issues of tax myself.

The guidance says that an employers letter should include gross and net wages. But my employer only paid me the gross - so the letter can only state the amount paid (gross, not net). I was going to have the employer write that I was left to deal with my own tax direct with the HMRC.

I have letters from HMRC showing that I declared income and paid tax (by adjusting my tax code from another job) - but I'm not sure whether this will be accepted? Anyone else have any idea?