Thanks guys, Keith in particular, you well and truly saved my bacon
I'm now back safely home just as it started pissing with rain. I might have figured it out eventually but being unfamiliar with the V6 Auto I had a bit of brain freeze and assumed the worst that the flashing R was some sort of gearbox doomsday signal

(Yes I have read about dying gearboxes in V6's so I saw everything flash before my eyes...)
No, it doesn't appear to be a faulty switch on the brake pedal, once I got it working again I've figured out whats happening - as you say if you move the shifter half way between reverse and park the symbol will start flashing, and yes the park contact on the shifter switch must be a bit dodgy, so even though the lever was fully in park the contact was not making thus it thought it wasn't sitting properly in park.
In this state it will not engage the solenoid when you press the brake, so there is no way to move the shifter back and forth to get the contact to register again, thus you're locked out. After manually operating the solenoid its all working fine again. I'll add checking and cleaning the switch contacts to my to-do list and keep a screwdriver in the car in the mean time in case it happens again. (Luckily I had a full tool kit in the boot today)
One thing I've noticed though, perhaps my paranoia again, is that the gearbox seems to be hanging onto 1st gear to a lot higher revs than before with gentle driving - around 2500 rpm or so instead of about 2000 rpm, (no, sport mode is not on) but I did disconnect the battery when troubleshooting the flashing R, so is it possible that the auto adaptive feature in the gearbox has reset to defaults and will soon "learn" to shift at lower RPM again ?
Thanks again guys, forum to the rescue, and this really reminds me why I love having a smartphone
