Just a note on compression, my 2000 105 series engine (S2 Spider) had low comp on one cylinder, about 25% down on the other three which were within a few psi of each other. With the engine still in the car, the oil-in-plughole compression test brought the compression up to the others so I guessed at worn rings or liner.
There is a 15 year old bit of paper probably under a cupboard in my garage with the psi figures on, but I guess it's not critical to my point, which is that the problem liner looked as good as the others upon removal, nice smooth sides, no steps or excessive wear etc, as did the rings, but the liner was worn in a shallow 5mm wide band, just around bottom dead centre, I guess this was allowing the pressure to escape. The overnight oil-in-cylinder test wouldn't have shown this up unless the problem cylinder happened to be at b.d.c., you would really need to turn the crank to run the rings past the worn part of the cylinder to detect any leakage.
I tried re-honing the liner as the worn area was very shallow, but it didn't get close to removing it completely, so I replaced with a 2nd hand liner. I now have a 105 series pen holder on my desk !