An update: I just managed to borrow a Mk1 Focus and plugged in a spare dash pod into the obd port via a flying lead to the plug on the back.
The speedometer works - and that's without any wires connected other than power and OBD!
So that's proved 100% that the speedo is connected via OBD
So why the heck doesn't it work in my car when everything else on the dash does? VSS Speed sensor compatibility (this is a Ford Sierra box)? - but I get a speed reading via my Torque/Bluetooth connection. Just now I even checked it wasn't getting it from GPS
Argh - so near and yet so far......