Right! I fixed it. I swapped the AC slider for the BD one, and now the
kick drum works fine. I can no longer control the amount of accent,
but it still works, it's just always on full. Great, very simple
operation. Strangely, the accent control was mounted upside down
compared to the rest of the sliders, i thought this might cause a
problem, but it didn't, i just stuck it back in the same way. Neato.
So, if anyone else finds a similar problem, see if the volume control
has failed.

Thanks for the help chaps.