I think the boiler has to come off the wall. You can't just remove the service valves and hot outlet union from the back. On the wall jig there's a "locking" plate that screws on and holds all the isolators in place. Once that's off the hot outlet union is pulled forwards through the jig. Great design eh.
Yeah I can see it now how it works now, thanks. What a pain.
I'm guessing that it's probably a good idea make sure that the all the o-rings are replaced once it's off.