The detachable bit and wiring are now standard on the LR4 but no so on the LR3. If there was a hitch when the vehicle was traded in, most dealers remove it and sell it back to you for about four hundred dollars, or new, about six to seven hundred; also there is the trailer light wiring harness.
The link below is to some files related to the hitch and hitch wiring.
DISCO3.CO.UK Photo Gallery - NAS Trailer Socket Pin ID and Wire Colours
I would recommend you stick with the official LR wiring harness as the harness is an extension of the trailer sensing bits that tell the air suspension to sort of freeze up as well as telling the backup sensors to shut off and some other computer related towing stuff.
If you are purchasing the detachable thing, buy the short one for the LR4 rather than the longer plough type of the LR3. Also be wary of eBay on the old style ones as LR did an upgrade or two on the locking mechanism. See the file link for details.
If you are going to tow a lot and clearance is not a concern, consider a Curt bolt on - cheap and simple - nothing can go wrong and what we are used to.