OK. Finally did the deed - Full to empty with a spare can in the boot.
On filling the cruising range showed as 432.
After 100 miles, cruising range had dropped to 335 - pretty much in line.
After 120 miles, cruising range dropped to 210 - wha?
Kept going.
Full light finally came on at 380 - range now 30 miles.
The last 30 miles are always pretty forgiving so it took until 425 for the cruising range to fall to 0.
Kept going.
Finally made it to 468 miles before she started to splutter.
Interesting to note that adding the 6 litres from the petrol can did not reset the distance after re-fuelling or the mpg - obviously below the minimum that whatever sensor that is inside the tank can read.
PS. Took 10 more miles to get to a garage and then 70 litres to fill her back up again.
Before the questions start:
Of the 468 miles, approx 250 (Leicester/Swindon/Nottingham) were Motorway - 75-85 ish (or stationary)
Of the rest, mixed driving, mainly Grantham - Nottingham centre.
Only problem now is that I can't remember why I set out to do this!
Going for a lie down now.