When I test fire I get noise but no burn anywhere, not even at the first mirror. This may not be connected to the dodgy power dial because as far as I read the test fire bypasses this and sends full power to the tube.
Do you see the plasma on the tube? Like a pink/red color when firing?
OK I will have a look …
I see lightning between the red high voltage lead direct to the case, looks like it is bypassing the tube!
I believe they are sealed on with clear silicon which was already blackened at this spot when it arrived.
you should try to redo connection. get rid of old silicone. reconnect and new silicone.
So no color inside the tube when firing?
someon youtubes this exact thing https://www.google.es/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0ahUKEwic9fu76qrOAhWGcRQKHTPNCmMQtwIISDAG&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DIKlPz9qI_ho&usg=AFQjCNGSccGcYtjNuDXvmZwv2jN6mrczAw&sig2=2erG9Z_1Yo-lqz_EHnVIlw
You will find here some old post with arcing, you aren’t the first nor the last ;-))
the power isn’t going through the tube at all, presume at minimum I now have to disconnect and reconnect both wires and silicon, or is the tube already dead?
I can vary the strength of the test firing by using the controller pot
@Frank_Fisher When you test fire, check to see where it is arcing. If it is arching where the connection is made at the tube, redo the connection. If it is arcing somewhere along the wire (which happens) due to a bad HV wire, you can try to seal the puncture with silicone, and try to move it so its not so close to metal. DO NOT fire it too much when its arcing, only a bit to test where its arcing. This can kill your PSU.
yes the pot needs to be quite high on mine to make it arc 
this is certainly not good. I don’t think it can kill the tube, maybe the PSU if not correctly grounded but it is verry possible that the arc connects the +15000v (dunno exactly how much on a 40W tube) to the metal of your machine.
I think the problem is at the connection as it has blackened, melted and bubbled the silicon there. I would have thought if the tube was working it would be so much easier to go through it instead?
Disconnect, discharge, remove silicon, clean, reconnect, apply silicon
OK that will be a job for tomorrow as I have no clear silicon. Which type of silicon should I use, the acid or neutral?
the unit should have come with a small tube of HV silicone that they use on the tube to insulate the connections…
Clean it up, reconnect the anode wire, and apply some and then let it fully cure…
If that doesn’t fix it…
You already have a damaged POT… now you are looking at a possible damaged tube… I wouldn’t be surprised if this unit was hurt during its trip west.
Contact the seller asap and ask for your money or a replacement.
The pot may not be damaged. It may just be that the air gap is large enough to require full power before it will arc.
Already contacted them but they say they only answer in a one hour window around midnight they say. Will triple check there is no silicon but never saw it on my unpacking.