Thanks to everyone who has contributed to this thread - I am pleased to say that the problem is now sorted.
I did replace the tube from the top rear brake light to an accessible location just behind the rear seats. I only undid the loom and removed the old pipe around the flexi- connector between car and tailgate where space is tight. Elsewhere I left the old pipe in position and ran the new one alongside. Threading it through was not as difficult as I expected.
Andy has commented how the old pipe is very hard and inflexible. This was not what I found in the region of the top brake light. However I then found that after a couple of inches the softer pipe was joined to a harder pipe. Well at least that gave me a connector to use...
As expected the region of damage was in the vicinity of the flexi connector. An obvious weak point, particularly when a hard pipe, containing frozen screenwash is flexed when opening the boot during the recent weather.
So thanks again for your assistance,
Andrew56