I wish I knew the answer, but I don't have a disco.  Would assume though that it is the same as removing one from any type of car.  Move the seat right back and you should see fixings at the front of the runners, slide it forward and you will see fixings are the back.  Undo these and the seat should lift out.

You may have electrical cable to undo first, look around carefully, under the middle of the seat and when lifting out, do it careful to give you chance to undo any cables you missed