Done a load of research.
Yes, I could get a second hand door, but it would have to be local. The postage is silly money.
I spoke to someone who repairs old DG and he said he doesn't touch anything that old and neither does he know anyone who does. I did find a trade supplier who could supply a replacement sliding door at an exorbitant rate, so I decided to bite the bullet and get a new modern door.
My first idea was to look at Screwfix. But the door I wanted was only in grey not white and the reviews were not brilliant. Plus, I couldn't find a fitter who was FENSA.
Next I went to a local firm who wanted the best part of 25 hundred for a 6 foot wide patio door.
Finally, I went with a Which? trusted trader for 16 hundred.