Thanks very much.
We do indeed have options 66 and 67 set.
It is working fine for all other desktop/laptops we are trying build, just the Surface Pro 3.
I was under the impression that these were required in order for the PXE boot function to operate (as it runs from a different server to our DHCP server) - It also mentions something similar on the link you provided - "When using DHCP Options for PXE Boot, Option 66 and 67 are needed. Option 66 must be the IP-address of your WDS server, Option 67 must be SMSBoot\x86\wdsnbp.com (which is the first file needed during the PXE Boot process)."
Is this not the case ?
Thanks