After a long, long delay I finally got 'round to snatching a day's work on this one.
It's either getting thoroughly out of control or it's very nearly finished! Answers on a postcard please. No doubt my poor melting eyeballs will be less addled after a good night's kip and I'll be fit to attack it afresh.
(NB: it looked like this twelve hours ago, so it's definitely going somewhere:)