Hi Peter,
Take a look at pendle-bike.co.uk.
We got a rear mounted rack from them for our Disco 2. The rack arms were offset to clear the spare wheel.
Since fitted to our Disco 3 and it works fine, but with 4 bikes on, it does restrict turning circle.
Thinking of contacting them to get a new set of arms without the offset.
The mounting plate is sandwiched between the tow ball and the Disco tow bar plate, so no good if you have the removable style tow ball mounting.
We also use Thule roof bars with two Thule bike carriers mounted to the Thule bars. They also work fine, but even with the Disco suspension lowered it's a challenge lifting the adult sized mountan bikes up there! Advantage of this though is that you can still open the rear door, which you can't with the Pendle rack.