Most will give a discount depending on your employer.
Just a matter of picking the phone up and asking. I've found it best to ask for their best price and then enquire about the airline discount. This way you can actually see what sort of discount you 'really' are getting.
APH, Purple Parking, BCP etc usually offer discounts depending on the employer. Most competitive varies significantly depending on availability, notice given and length of stay.
Hope that helps.