That's a bugger!
These have some information (quality unknown) on some of the codes Disco II ABS Faults
I actually haven't used HDC, I tend to go low range, manual mode and use the actual gear along with a bit of brake pedal action if really necessary. So, you can still work around it, but probably not quite as good without the central diff lock.
You *may* get some luck with the relay issue if you can find the right bloke who can fix it. Chances are the relay is a mosfet or other common item and can be replaced on the board. Of course knowing which one is the dodgy one could be the issue. I've never looked at the D2 internal fuse board (fingers crossed), Rave and tracing circuits might point out which one.
EDIT
Looking at the circuit diagrams and R222 on passenger compartment fuse box/panel is the one. Though you'd need to buzz out the tracks and the output (GREEN/black) which goes to the 3 rear lamps and the trailer socket. It could be a short in the wiring back to the lamps.
I think it's probably going to be a short on the wiring if I had to put a quick bet, but finding it will be fun.
Pull the wire off that feeds out that signal and check your brake lights still work normally, if they do then leave off and try HDC again (if you can....) it may be a running fault for the relay registering open circuit since it only turns the brake lamps on and off. If you want to get really clever, put a load resistor on it to make it think the lights are attached :)

