Hi CN, thanks for you help, I really appreciate it. Support told me that the error that I am getting when configuring the dhcp relay is not expected behaviour and that I should be able to add this even with only a default route advertised from the hub. They've passed it to the engineering team to look at and provide a resolution. They say that they can add the dhcp servers in the backend manually to resolve the issue, but I'd need to contact support to change, add etc, so obviously not a scalable solution, especially for numerous spokes.
They also said as an alternative they can disable hub/hub tunnel formation in the backend (was this was you were referring to previously - I thought that was just disabling the readvertisment of of hub local networks), but I don't know what the knock on effect of this would be. I guess hub/hub tunnels are there for purpose and breaking that could lead to other issues. I tried to get out of the support person what this could be. He said he would find out, but didn't elaborate when he emailed me later so I'll follow up. Do you know any caveats to disabling the hub/hub tunnel, or if this should be avoided?