I got my letter from my GP after three weeks. I rang the surgery for an appointment, and happened to mention the letter. The receptionist came back to say it was waiting for me, no attempt had been made to ring me to say this. I paid my £15, and the letter was a direct copy of the draft I had taken from the PINNT example, but on surgery notepaper.
Next week I also got a letter from the hospital. This time there was no charge, and it had been redrafted in proper medical language.
I then tried to make contact with the airline Easyjet. First time was by phone, the operator agreed I could take an extra bag with my medicines either in the cabin or as hold luggage. She took my email, and I assumed I would get confirmation of this, but nothing arrived. As I wanted written confirmation, I tried again with an email, and also asked how to contact customs. Got a reply after a week confirming free baggage, but just suggested looking at airport website to find contact.
Manchester airport website has FAQs on wheelchairs and oxygen cylinders, but no way to contact them to ask general questions. Tried phoning, but no reply. Only way seems to use Feedback Form, have tried this.