The link actualy shows up on Manage Your Booking.
I only called the office as a precaution to confirm the address and time details as the chauffeur drive service has in the past not pitched up for my dad (atleast on one occassion).
I was told that although the link shows up online, passengers booked on that specific fare and have reserved a chauffeur online are notified 3 to 4 days before the flight that a chauffeur service will not be provided and to make alternative arrangements.