Shaggy Sheep Driver
...I've noticed static ports under the fuselage at the back, between the engines. Are these just additional ports for the aircraft's general static pressure measurement system, or do they have a specific function?...
I'm not the right person to be answering this, and the reference diagram I'm looking at, whilst very detailed, is not particularly clear - at least to a pilot!
However, from your description, I wonder if they might possibly be the two pressurisation static ports that are located in that area?