P&O supplies a shuttle bus between airport/wharf for those passengers who book their flights through P&O. Passengers CANNOT book seats on these shuttle busses, unless it's part of the air flight package.
HOWEVER, subject to bus seats being available, you can join the shuttle bus (a fare, paid in cash applies). Keep in mind shuutle busses meet only those flights on which P&O passengers are expected to arrive, and there is no guarantee that seats will be available!
On arrival from your flight, look for a P&O representative - he/she will be wearing a P&O polo shirt with name badge, and be carrying a P&O clipboard/sign (the sign may not be showing, if passengers are not currently expected). Alternatively, look for Murray's coaches outside the passenger terminal. Ask the P&O Rep, or coach driver (nicely) and you'll most likely be away.
However, repeating, availability of seats and presence of coaches to pick up passengers is a "must".