I want to simply pxe boot a workstation on a different site and have it
contact our central site to restore an Image. I also want to do the
same on our main site that is split into several vlans.

Currently booting machine in the same vlan as you would expect is fine.
I have a linux server running all the zenworks imaging plus the proxy
dhcp but I have a seperate netware server running DHCP.

I was hoping I could just put a dhcp setting in that would forward any
pxe booting machines to go to the server hosting the proxy dhcp address
and then the tftp server and so on.

I am seeing a few bits in the documentation that talks of setting
forwarders on the switches etc is this the only way as we have no
control over our routers?