Lets expand on this, it will be fun... especially since none of this is really Buell specific.
A customer has bought in a 2008 XB12s lightning, it has been imported to England
England huh? Isnt everything upside down there? Oh.. thats Australia. That changes everything.
has not been run for nearly a year, so charged battery but it only fired on one cylinder,
So, off to a good start but this indicates that you ran year old gas through the pump and injectors.
Also, which cylinder did it fire on? Lets assume the rear! Details are unimportant!
thought I would drain and replace the fuel then tried again this time
Sort of like closing the barn door after the horses were gone, but whatever.
tried a small amount of fuel direct into the engine,
Good. I would have done that too. +1!
nothing, checked and there was a spark on rear cylinder
OK, good! +1 more! But you did not check the front cylinder for spark??? -1 <sad face>.
plug looked black (carbon) so cleaned
OK, good! Not really necessary as we already established it was sparking.
then when I checked it again it is constantly arcing and there is a spark like a tesla arc which will jump a 10mm gap to the frame
Constantly arcing? Constantly? Sounds like the rules of physics have been broken? CoOter is constantly stealing beer out of my fridge and constantly never replacing it, so I hear ya.
To wit, why are you still evaluating the rear cylinder when the front cylinder is the one not firing?
discounting grey connector on ECU stops this.
Well, duh. Disconnecting the connector from the coil, as well as a connector from the battery will stop this too. To your point, I'm sure disconnecting the taillight connector proved pointless. This is really weird.
SOLUTION: Run 2 cans of Seafoam fuel system cleaner through the tank, perform a TPS reset and bleed the rear brake. That should solve this. If not, you must part it out.