This evening I'll edit this post into a big version, but a spoiler is that it was addressing the oil leak, quite a significant one, mentioned on the last few MOTs as an advisory "oil leak front of engine". I'd already guessed at the culprit, and even headed down to the car on my first visit armed with the necessary gasket, since the replacement involved alternator removal amongst other things, and I thought it would be nice to 'kill two birds with one stone', while I was fitting a good alternator to get the car mobile anyway. I posted a video of the seal replacement in a related thread after doing a search for 1.2TSi oil leak on here, and coming across this thread: https://www.briskoda.net/forums/topic/438119-2011-12tsi-dsg-oil-leak-head-gasket-gone/page/2/#findComment-5931213 Typically in that vid, the lambda sensor removal is made to look much easier than ever happens in real life, in my experience. It does have to come out in order to get the heatshield off, annoyingly, which then gives access to the right-most screw that holds the filter/cooler/alternator bracket casting on. Wildly underestimated the work involved, and didn't even contemplate it on that occasion. Spoiler: this figure of 8, plus another loop, rubber gasket seals the casting that holds oil filter and cooler onto the front of the block. It had stopped sealing it. The rubber was hard as a rock, squished flat with no compliance left at all. Can't blame it at 13 years old and 159k miles driven. Not the best design, possibly. There were also a few tiny bits of casting flash around the edges of that face where the seal sat, maybe holding it off the block a smidge more than planned. Tidied up before new gasket was fitted.