I formed the impression that virginblue needed to get to the airport from the city centre. The Park & Ride cannot be used for that, as far as I know, because you would not be in possession of a ticket when you got on the bus. I believe that tickets are issued only on boarding at the car parks themselves.
Indeed, hence my confusion about the P&R option as I am without a car and starting from the wrong end (I will be arriving by train).
I guess the best option is to take one of the local busses from the city centre to somewhere near the airport.
Quite surprised how difficult NWI makes it passengers to use them - 10 GBP "development fee" (given the recent history it is more a "winding-up fee"), no proper public transport...