as I can raise a letter of complaint?

Try this link:

https://www.apple.com/feedback/

Ciao.

it appears Apple doesn't want complaints, only feedback. I have a complaint about that in itself

Apple Store Customer Service: 1-800-676-2775

Corporate Address:

Apple

1 Infinite Loop
Cupertino, CA 95014
(408) 996–1010

is there no provisions for written 'feedback'? I have a complex-ish complaint that I'd prefer to have in writing.

Feedback and complaint is the same thing. Other wise call the number you were given.