A few years back when they both visited the states post wedding think it was 2012, they had a ride up to the LAPD ASTRO Div and hitched a ride in a Grand that was parked up there (escorted by the CHIPS only Bell 407) so maybe it was that impressionable on Will and his mother decided to authorise purchase of one?
Think said Grand was the same one, the Hamster went in when he was doing one of his documentaries in the states, not a Top Gear one.
Cheers