There was a time where the Disco did not have diff locks and it sounds like yours is one of them. It will have TC instead.
Land Rover dropped the Difflock in favour of Traction control on the TD5
but after customer complaints it became a dealer option on the new cars.
certain TD5 gear cases have the difflock inside but no lever, you can buy the lever from ashcrofts if you want to add difflock
cheers thats a big help matehave a look at this: Ashcroft Transmissions
yes it has traction control, but it seems crap in ice, my 300tdi had no trouble going up my drive but the td5 doesnt want to.
Traction control is better than diff lock (here comes a big debate)