The dealer has removed the head and two pistons (the cylinders that reported loss of compression).
The Rings, pistons and cylinders do not show any wear and measure within the spec limits.
They are now testing the Turbo.
What I cannot understand is that the reason that they perfomed a compression test is that they noticed blow back through the oil filler spout when the cap was off - my understanding is that the only source of this type of blw back HAS to be via the rings / piston / cylinders?