Drops of oil under the bell-housing of the gearbox are often diagnosed as a gearbox seal failure, and this is often the case. However for a true diagnosis, you will need to look closely at the type of oil leaking out. Oil with a reddish colour will be gearbox oil, and will require the gearbox input shaft seal to be changed.
Golden/brown/black oil will be engine oil and will require the crankshaft seal to be changed.

The gearbox will need to be removed, so you can either use the gearbox removing guide for more detail, or follow below for an overview.

Parts used:

• Crankshaft seal kit A 160 997 02 46

Start by removing the rear panels, crossmember and exhaust.

Unplug and remove the intercooler fan.

Remove the wheels and remove the hub-bolt using a 13/16 double hex, or a 21mm double hex socket.

Disconnect the intercooler pipes, and then remove the intercooler itself.

Use an E8 to remove the 2 small bolts securing the intercooler ducting, then remove - you may want to split the ducting in two, in which case remove the 2 small spring clips.

Pop the driveshafts; they don't need to hang as when the gearbox is removed, you can manouevre the box out with the driveshafts still attached.

Remove all of the gearbox securing bolts (E12).

Support the gearbox on a jack and remove the gearbox mount (E12) - remove it from the gearbox itself instead of the chassis as it gives you a little more room to move.

Unplug all of the connectors to the actuator, selector motor and position sensor, and remove the gearbox. If you pull it away and down you have enough room to drop the offside driveshaft below the chassis tube; you can then move the gearbox down to move the nearside driveshaft free.

Remove the clutch cassette using the access point at the rear of the crankcase.

Use a 17mm double-hex socket to remove the flywheel mounting bolts.

Chock the mount to stop it from moving if required.

Remove the flywheel mount. Note the locating lug: the mount will only fit on 1 way.

You can see the drip of oil coming from the base of the seal.

Remove the old seal, pierce with a thin screwdriver and lever out. Do not lever against the mating face!

The crankseal fitting kit comes with a useful locating tool to ensure the seal goes in square.

Place the guide over the end of the crankshaft (it is a snug fit), and slide the metal ring and seal down the guide so that the seal starts to locate. Press in firmly until the seal is held in without support.

Remove the plastic guide (should just slide out, leaving the seal in place), and place the metal ring back onto the seal face again.

Gently tap the metal ring evenly all the way around until the seal face sits flush or just below the mounting face of the flywheel mount.

Now reattach the flywheel mount - you may use a small amount of threadlock if you wish.
Refit the clutch. Note: this will also only fit on the mount 1 way due to the locating lug.

Refit the gearbox, and test.
Keep an eye out in the following few miles for re-occurence of the leak. There may be some residual drips but wipe these clean and see if they re-occur.