Well, there wasn't any issue with the brake servo, the car was legitimately loosing vacuum somewhere else...
now I have a few idea's, but one thing stands out.. When i removed the hose to the brake servo and blanked it, the lean condition remained. When I released the vacuum hose and allowed it to leak, the engine obviously nearly stalled, but, to my surprise, the fuel trims did not change... hardly at all!? I found this a bit strange.
I put the hose back into the brake servo, put everything back together and tried an old trick. I removed the connector from the MAF sensor and went for a drive. (To force the engine into limp mode)
She drove flawlessly, no hesitation, no lack of power, no strange idle, no hard brake pedal, which leads me to believe a sensor, somewhere, isn't functioning correctly, but also not causing a fault code. This should be fun.... Obviously this isn't a solution, fuel consumption will go up (I hope) and you run the risk of permanently damaging your catalyst (engine will run rich) but for now, it's much nicer to drive.
Time to start diagnosing the many sensors that make up the engine management system.
Nick