Standard boot of PXE (x64 image) was failing. After initially connecting, I got a brief "Preparing Network Connections" before it rebooted.
The driver pack for this Laptop does not contain the required drivers for PXE!
It's the one listed here http://ftp.hp.com/pub/caps-softpaq/cmit/HP_Driverpack_Matrix_x64.html
Useful to download and inject the drivers into your build image, but useless for the PXE boot requirement.
1. Download and unpack the latest drivers direct from Intel. I used this link fir the I219-LM driver.
Extract the drivers and import into SCCM via the usual method.
Select all the NDIS64 drivers and import to your Boot x64 boot image.
Update distribution point if you didn't tick the option when running the import driver wizard.