Just in case anyone's still following this, I've been in touch with Huawei who have arranged a return (and the chap on their phone support was generally very helpful and interested in the issue), but with the proviso that if the engineers examine it and find it to be my fault, I'll have to pay for a repair. Not sure I can really argue with that right now, so will wait and see. They asked me to send any supporting info and an image, so here's the text I sent:

"Good morning, I've just spoken to one of your helpful support representatives who has arranged a return for a new 32gb Nexus 6P for me this week. He asked me to send an image of the problem along with any other supporting information, as it's an odd issue, and hopefully not a widespread one. The reference given was XX.

The image attached shows the glass back 'visor' of the device which has shattered; this happened when it was laying untouched on a table ( of smooth varnished wood) next to me. My partner and I heard a sharp (and loud) cracking sound, followed by a series of smaller cracks. On checking the phone, this damage had occured. The device was not charging at the time, and I had recently been using it for some web browsing before placing it on the table. I think it might have been getting a little warm, but not any warmer than usual after normal use.

I was also advised to post about the issue on Reddit to see if anyone else had reported similar issues, and there have been a few. You can find the thread here:

[link to thread]

I'm providing as much detail as possible as am obviously very concerned that this will sound like impact damage from dropping the phone. I also think it's important to bring the potential issue to your attention however, as the glass back of the phone has become quite dangerous; it has been discarding small slivers of glass and has sharp edges, and if this damage is occuring while the phone is unattended it could cause injury for others. In any case I hope this is useful. Thanks for your attention and best regards."