The plugs looked fairly consistent when I checked them last week, so it sounds like a compression test is the next course of action. I’ve already checked the inlet piping and can’t see anything obvious.
Despite having fairly amateur mechanical knowledge, like you I prefer to stick with tried and tested diagnostics but this car has me stumped! I haven’t checked/replaced the lambda or cat yet but I suppose issues there would reveal themselves in the MOT result.
I bought the car with an inheritance as a bit of a gift to myself, so more than anything I think I’m just disappointed that my seemingly well looked after example is proving so troublesome! ☹️