There was some glass inside but more outside so I guess it was most likely some sort of stress failure. Found a replacement on Ebay for £45 so hopefully all good soon.
Well that was strange. Finished the Kona AA today and went for a quick 30 minute ride to check brakes and gears, all working well and the bike rides really nicely on the road. No doubt the aluminium frame with the P2s will make it a bit of a bone shaker off road but I have my 853 framed Explosif...
For completeness here is the finished article. Seatpost ridicuously high as per usual with Konas, plus this is not quite my size but I'm trying it out later today.