The clamp [red arrow] has to be undone. Towards the rear of the car, their are 2 nuts holding the exhaust to the sub frame rear- undo these and you can then move the exhaust from side to side. this helps getting to some sump bolts.
The front down pipe/cat will almost certainly have to come off. Once the heat shields are out of the way things become clearer.
That pipe [blue arrow] will just pull to the side, pay special attention to the twin nut and stud arrangement.
There is no gasket, a black RTV silicone is used. Google Pro Seal N80046.
At first it seems that the sump will never shift- but by tapping in a series of thin wedges/screwdrivers it will drop off.
Whilst there- look very carefully at the mesh filter up inside the oil pickup. Any rubbish trapped there- stop spending money
Main tips- no special tools needed, photo everything at every stage, allow plenty of time.
My experiences are from a 1.6 Hdi C4 picasso- but I doubt there will be much difference