I too have suffered from the demand for a 'GP's letter', in my case, for reassessment for a hearing aid (GP, 31 August).
I found that the following made the author realise how I felt: 'Re: your ref
You ask for a letter. This is a letter.
After two or three of these, I got a phone call from a real live person and no more letters were requested.
Dr Brian Mansfield, Beckington, Somerset.