We have reached a resolution and I'm satisfied with it.
The frame was damaged in an incident I had completely forgotten about. The chain was thrown off the front cog about 2 years ago, onto the frame, smashing off the metal guard bonded onto the chainstay. The frame was weakened by this. I didn't think any more of this at the time as all seemed to be ok. So, its damage not covered by warranty. The warranty covers material or manufacturer's defect.
I sent the photo of the damage to 2 separate composites specialists both said it was not repairable. So I need a new frame.
After a lengthy discussion with Canyon they have offered a new frame under their crash replacement scheme at a reduced cost. They have been very fair about this as they only offer it to bikes up to 3 years old. Mine is older. They didn't have to offer this. They have also given me a new set of forks free to match, as the colour is slightly different.
They have been really good in responding to my many and varied questions and generally pretty fair in this whole business.
So, overall I am satisfied, all things considered.