I got home late today but started tinkering (just try and stop me!).
Found the pressurstat pretty easily (on top, under the cup warmer tray). Wound it down to about 1.0 bar (theres about 0.1 bar hysteresis)
Pulled the covers off and went looking for the overpressure valve. Eventually found it hiding down the back under the tank, at such an angle that is impossible to access with a screwdriver. At least, thats without bending some copper pipes... Pulled a few shots to dial it back to 9 bar (from the factory 12 bar!) In those trial shots I really noticed more sourness at the lower boiler pressure and pump pressure, compared to yesterday. (ie: Wow, even I could tell the difference!)
Finally made a cappa for me and the wife too, nice and easy. The lower pressure really takes the edge off the steaming power, still fast enough though. Yum.