As Nodge says, with the car in neutral...
1) If the switch to enable HDC is turned on, does the green HDC information light flash? If not, the wiring to the switch is faulty.
2) If you then engage 1st gear, does the green light then stop flashing and stay on solid? If not, the 1st gear detection switch, or the wiring to it, is faulty.
Do any of the HDC, TC or ABS amber warning lights illuminate either when HDC is not selected or during when HDC is selected? The '3 Amigos'. If so, a fault has been detected in the ABS system and you need to read the diagnostics codes to find out what the problem is. Do the 3 Amigos light up when you turn the ignition on and then go out soon after or when you start moving? If not, the bulbs have been removed from the dash console or you have some other fault.
Assuming you do not have any of the above problems, HDC should function. With HDC enabled and in 1st gear, HDC will (spelt should) maintain speed "relative to accelerator position" (as described by my '99 handbook). It does not say what these speeds are, however the Rave workshop manual says they are 6mph in 1st and 4mph in reverse when the throttle is closed. The forward speed is reduced to 4.4mph on rough terrain and if the engine is cold it is increased to 7.5mph for 1st and 4.4mph for reverse.
HDC will not function if you press the brake or clutch peddles.
You will need a steep slope to test HDC on because the car will need to want to naturally move at greater than 6mph - that's especially true for a diesel manual. A petrol will not give so much engine braking, and I'm pretty sure an auto, even in 1st, will not provide quite so much. With your feet off the peddles, HDC should maintain the speed at about 6mph. You should feel and hear ABS in operation - you definitely can with my WABCO ABS pump, not sure about your Teves pump. If there is someone behind you, they should see the brake lights illuminated because HDC energises the brake light relay when its in operation (according to Rave).
If your speed rises much above 6mph and there are no signs of HDC operating, then the brake and/or clutch switched could be faulty - however, these would normally throw 3 Amigo warning lights - as should any other reason for HDC not operating.