Ok, I've managed to find the solution. I was originally using the answer provided in this post:
Which said to use this config file:
root@raspberrypi:/etc/dnsmasq.d# cat 10-tftp.conf
dhcp-option=66,“10.0.0.239”
dhcp-boot=pxelinux.0,10.0.0.239
However it obviously was not working.
Solution:
root@raspberrypi:/etc/dnsmasq.d# cat 10-tftp.conf
dhcp-boot=pxelinux.0,,10.0.0.239
10.0.0.239 is the PXE server.
For some reason, the double comma is not coming through on the post. Should be 2 commas between the 0 and 10, screen shot below.
Issue resolved, thanks.