**UPDATE** This seems to be a bit of a work in progress. Seems like if (as I did) you set the dock up as local admin, after following the steps below, it is necessary to login as that same local admin account before the preferences get pushed to other network users. In my defence, it is Friday afternoon and I should've left 8 minutes ago 😝
Found another way to do this:
1) Setup dock icons for local user on one mac,
2) Copy the ~/library/preferences/com.apple.dock.plist file to OD master
3) (maybe optional but just in case) increase Everyone rights on file to Read Only
4) in pain-in-the-*** Profile Manager, configure a Custom Settings payload for device/device group
5) Click Upload file and choose previously copied plist file.
This doesn't duplicate any icons added via dock preferences from what I can see, but it did require a couple of reboots to enforce it for network users.
Next year I'm tempted to just take all the macs off the network and let them do what they will........!
John