Can also relate to sexual favours offered by a hitch hiker to one who offers a lengthy ride with minimal questions asked.
Can also relate to sexual favours delivered by a prostitute to a John.
Oi Gary, get the car keys, I'm gagging for a spot of the old Cock au Van.
Listen lads, I need to use the company vehicle this weekend. Ive promised Rita from the the sweet shop a bit of Cock au Van.