There should be two studs on the bottom of the tank, with nuts on the underside of the body. Correct on right side, it's to balance the driver, which is almost always LHD on an S1.
Mine came out easily with the head off, though muscling the old rubber hose was tricky, I opted to saw it in half and remove the remnants from the filler neck and inlet after removal.
Dakazam, I have my engine and tank out currently if you need dimensions from that area, already did some work to modify a left side tank from a cheap eBay one.