It goes all the way across and although I didn't test my theory too rigorously a small amount of pressure did appear to show it would widen the crack.
So, I ran superglue across the length of the crack and hopefully it will 'capillary action' [pun intended] into the crack and bond it back together.
I added a quick coat of matt varnish blend it in and also tried to cover up any paint chips on this and the other towers. It was always the risk of choosing the clay over Milliput but it's been OK for 6 years and should be good for now. It's not like I need it for anything at the moment and even if I was gaming I don't think the Skyshield Landing Pad would be high on my list of choices... Anyway fixed.