no problems with the battery - starts up fine.
I have a volt meter connected to the cigarette lighter in between the seats.
when i first climb in, it shows between 12 and 12.6 depending on when last used and for how long a trip.
when cranking, voltage drops and i have seen as low as 7.5.
if battery was low, volts then climb to 14.6 or 14.7.

all quite normal.

now yesterday I noticed whilst cruising up the M23 (had done about 100 miles up till then), i noticed that when under power volts were showing 13 or 13.1.
when i took foot off gas, volts went to 13.8.
foot back on power then volts back to 13 or 13.1.

any idea for the voltage fluctuation?

Not having worked much on the electrical side of the FF yet. I can't really say if these figures are normal but, as for when they happen it seems like normal working voltage spikes and drops.

The battery should hold a residual amount of over 12v
Once you crank the car over loads of current goes to the starter motor to turn the car over. ( that's why continued turning of the car over drains your battery faster then anything )
Once the car is running the altinator will be producing over 13v to charge the system while it is in use.
And I believe the accelerator fitted is a resistance version, so when you press your foot down the sensor in the peddle creates a difference in resistance that sends a signal to an ECU letting the engine know what demand is being requested. So when you take your foot off the pedal resistance goes, and you have a higher voltage.

I think  Bushy

2003 RR Vogue Monté Carlo Blue

After starting the battery wants 14.6v to replace the used power for starting... An hour later, the output will fluctuate around 13v as the battery will be full by then and the alternator just keeps up with demand...