Update: the Adaptil spray didn't work.
On Monday morning I put him in the car and he started barking before I'd even closed the boot. Exasperated, I took him out, back into the house, back into the kitchen behind the baby gate and left him five minutes. When he'd calmed down, back in the car. Bark bark. Took him out, repeated the process. All the while saying nothing at all. Fourth time we did it he got into the car, gave me a searching look, licked my face as I settled him into the crate and was silent for the whole trip!
Quiet again this morning. Fingers crossed we're on our way to breaking the habit.