Update from previous post. I have submitted a letter to my GP, but have received no reply in two weeks. Have had to give this up as a bad job and tried to get a letter from the hospital. Lets hope I get more luck here.
Have managed to get travel insurance. I spied a comparison site in the Sunday paper www.medicaltravelcompared.co.uk . I filled in a lengthy form and got a list of potential insurers. I selected an outfit called www.lessclicksmorecover.com for a reasonable price, I just hope they cough up if something goes wrong. The main difficulty is that it is difficult to match your strange type of cancer to an exact fit on their list, but I have made the best match possible.
I have discovered that Nutricia, my feed supplier, will provide a heavy plastic box for my medication. This weighs 4 kilos and measures 37 x 88 x 46 cm. I am not sure yet if I will use it and if the airline will charge extra. If I can get a letter I will get in touch with them and see what is possible.