That's the third version I've seen of this board! It looks quite different to mine in a lot of places but it would seem the circuit is actually very similar, just a different layout.
The cap values are invaluable, thank you for that! I'll update if I can get this thing doing what I want it to
Regarding your tube that's bad luck, I would usually suspect the tube last. Sounds like one of the guns has had it.
You can get (quite expensive now) a tube rejuvenator but you may be better off sourcing an Acorn AFK-50 monitor as the tube should match. Actually they are good for spares as they are almost the same monitor.