Ok, so I fixed my own problem.

You have to set a predefined option on your DHCP server. If you are using DHCP and WDS on the same server, it does this for you automagiclly. If you are running DHCP on a different server from your WDS server then in the DHCP mmc you have to right click your server name, click Set Predefined Options, Click Add; Name: PXE Client; Data type: string; Code: 60; click Ok.

Right click 'Server Options' in the left field and click Configure Options, click Advanced, check 060 PXE Client, and type in String: PXEClient (no space between PXE and Client) and reboot services.