Thanks for all your help guys. Finally got it working. It was the draytek in the end. They seem to be a bit odd with DHCP option 66. Setting the option from the web interface wasn’t working so in the end had to do the following from telnet:
srv dhcp option -e 1 -1 [lan number] -s [Fog Server IP]
not sure what the -s flag is but it worked after that.
Couldn’t find anything about this on the net so hopefully this will help someone else in the future!
Thanks again!