Recently, I bought all the filters to do a full service on it. I was going to do the job today. Took it for a 10 mile drive, to get the oil hot ready for draining, and parked in the garage. Jacked the front of the car and laid on the floor, looking for the access flap in the undertray below the sump plug. There isn't one.

In over 50 years of DIY servicing and repairs, it's the first time I've been beaten. .........And I've even had engines out and stripped, re-built, and re-fitted them.
Would it have been so difficult for PSA to put an access flap in the undertray, like my F25 (and the E46 that preceded it) has? To be honest, the fuel filter and cabin filters both look like a nightmare to change as well.
Rant over.