Where is the upstream O2 sensor?

[MartynG]

I've an old workhorse 2008 Mk3 2.2 D-Cat T180 that's starting to have a few aches and pains as you'd expect after 150,000 miles, but the most annoying is the "usual" Engine Management Warning Light accompanied by the VCR fault and 4 wheel drive fault.
People much wiser than me have told me to change the O2 Sensor Bank 1 Sensor 1 which is upstream of the catalytic converter BUT under the bonnet of the T180 d-cat is so crammed that everything has had to be shoehorned it (changing the alternator recently was a nightmare).
You can see one sensor from underneath, and I've changed that one, but that appears to be sensor 2 - so where the heck is sensor 1 buried?  There's absolutely no room to see anything behind the engine from either above or below without a lot of effort, so does anyone know a way to get to the upstream sensor on my car - wheel off and remove the wheelarch liner perhaps ?