Hi, a little confused...

I will be putting a bank charges reclaim (yes - I know my chances!) but have a few Q's before I proceed.

The account I want to claim on is a closed Halifaxicon one.
My current bank is with Natwesticon.

I sent for a preliminary charges list from Halifax, to which they replied that they would need further information to process the request.
I am about to write the response, but want to ask about Financial Hardship.

Do I fill out a Natwest I&E form and send it with my letter to Halfiax?
or:
Do I fill out a Halifax I&E form and send it to Halifax?

I want to mention FH, the Financial Ombudsmanicon, and the hardship code in my letter to Halifax, is there anyone who can help with getting the correctly termed letter together?