Gearbox assy down is the way.
If you are like most of us poor mortals you are not a "Superbeing" and will find it hard, even using a floor jack to lower, manipulate, balance etc that heavy unit beneath the vehicle.
Though I have not done the clutch in a Td5, I have done the clutch in our '98 Tdi300 Disco, for the job I made up a little gantry style device that's feet sat on the floor of the car either side of the tranny tunnel, a 40x40 RHS beam linked the two supports and a long 16mm dia threaded rod with nut on top of this beam attached via a short piece of chain and an angle bracket at the centre of balance on the top of the gearbox/transfer case. With the front wheels of the vehicle lifted about 200mm off the garage floor I was able to easily unbolt the gearbox from the engine, slip the gearbox/transfer back and lower it to the floor beneath the vehicle and remove and replace the clutch with a new kit. Most of this done from within the cabin of the Disco, with the aid of the screw thread and a well lubricated thrust bearing and the added safety of having the load supported from above and pretty well balanced front to rear and left to right the gearbox was gently lowered to the floor and then easily and accurately raised again to be easily slipped onto the back of the engine after aligning the clutch/plate and splines.
I am sorry I do not have photos of the device and it would not help anyway as I have no idea how to post them anyhow, I am away from home at the moment but will try to get photos posted some time in the future as this gear was easy to make and so simple and safe to use it would be of benefit to anyone doing the job and far less fuss than removal of the engine (IMHO)