Question on tachoheads in 3.5 Tonne

ROG:
As there are no break/rest rules for domestic regs then with two drivers sharing the time in the driving seat in one van the shift could be a long one because when not driving the passenger can count that time as not working or driving

Vosa (as it was then) told us that being a passenger in your company’s van counted as duty time. This might just be their interpretation, as I cannot find an accurate definition in the legislation of ‘duty’. Anyone know of a case precedent?