Hi Simon This truck sounds to me like its between 20 and 30 years old and is running on a mk seven sevcon controler
later models like the ES went onto mk10 cable form if it has a black box located on the R/H sie of the panel as you look at it then its a mk 7 you say there is 39 volts to the chassie with the battery disconnected this must be cleared then I sugest you check for earths on all motors and the system seems to be sensitive enough to detect this I know Because I had about sixty of these trucks to look after.
If my memory serves me correctly there are micro switches along the bottom of the direction contacts and shorting contactor these must be operating correctly disconnect the battery push each direction contactor in inturn and listen for them clicking sometimes they go out of adjustment or even go open circuit. when you do this ensure you push the wiper plate up flush with the directin coil bobbin plate. Remember safety first disconect the battery ensure the drive wheels are of the deck.
You are getting your Direction contacts in so you have proved your hand brake switch and foot control switches are ok. I think the problem is on the shorting contactor swiches I think you should check them all out


Titus