Leaderboard

I doubt it's an air leak. Or anything to do with the O2 sensor. But- thinking aloud, what makes you think it's running rich all the time? Are the plugs all fouled up with soot? Is there a strong petrol smell all the time from the exhaust? the sensors if ok, will tell you what the ecu is seeing. The ecu only bases it's fuelling on what it sees. What you see is what it sees (the ecu sends the live data to vcds, so you're seeing through the ecu's eyes so to speak...). Check the data from temps, map, O2, TPS. It should become apparent which one isn't playing nicely.

It's obsolete from VW now but the part number was 6X0 803 550 A and should look like a metal bar as shown in yellow below

Your injectors are fine if running rich. Something is telling the ecu to run a rich fuelling map. Usually temp sensor. But could be a number of things or combination. VCDS will tell you what each of the sensors is seeing, so have a look at each, cold and once warmed up.

No problem fella - probably missed in the MOT as it's in the boot. Battery gives of hydrogen when it gets charged - but if over charged it gives off hydrogen sulphide not a big deal in an engine bay, but is if its in the cabin. Just went looking for a PN, but no luck - its a bent piece of metal rod that slots into an opening at the front, sits in the groove around the battery and only one bolt -that you can see in my pic above - at the eye shown below: