Well...this is embarrassing...
I put the bike into dealer mode using the patented Suzuki paperclip and it just came up with C00 so no errors thrown.
Then I look at the battery and think "I wonder if that lead needs to be on the negative instead of the positive...it should have been all along. What a tit.
I now just need to get hold of one of those rectangular nuts and she's sorted. I'm relieved but also ashamed.
Strommer - thanks very much for your advice on this one - much appreciated!