K14 engine very poorly

[noggenfogger]

Hi all. My first post here and I'm asking a question about the ex wife's car that is a micra k14 that was manufactured in 2017. Its only done 28k and has developed a bit of a issue and I hope some of you may have more of an idea about the architecture of these engines.

So the story goes a few weeks back she calls me saying the car has broken down after she drove it for 22 miles with a misfire. When I arrived I found that cylinder number three spark plug had completly disintegrated leaving just the porcelain debris under the coil pack and the lower porcelain and the core was missing presumably mashed up and ejected. I removed the cam cover to remove what was left of the plug on the roadside, hoovered out the cylinder to check no debris and fitted a new plug and coil pack. The car started right up but did seem to have a bit of a tick it didn't have before. I rechecked for debris and drove it home. Over the last few weeks the tick has developed into a knock and yesterday started to throw a pre cat lambda fault. Lambda sensor is fine so guessing it's running rich, new plugs look nice and clean with no major evidence of running rich. Did a compression test and one and two are a healthy 215psi but number three is 150psi so well down. So oil or coolant loss and no major crank case pressure so thinking maybe a busted valve from eating the sparkplug. What's everyone's thoughts?

TIA

Tim