Just thought I'd give a quick update on this topic for future prosperity. It's taken some time, but approximately 5 weeks after placing our boy in a crate in the car, we are pretty sure the barking really is stopping for real this time. It's been slowing down for about a week...we didn't dare hope, we almost still don't dare hope, but we think we might have licked the worst of this. He had two half hour trips yesterday and today, and didn't bark once. Before car crating, that would have meant an hour of continuous barking.
Here is what we did: (i) did not interact with the dog at all when he is crated in the car (ii) left dog with a squeaky toy and bone and make sure it is fully engaged with one them before starting the car (iii) fed the dog in the crate in car once a day without going anywhere to get him used to getting in and out of car crate (iv) take drives e.g. to supermarket and back without letting dog out of car (to reduce excitement and anticipation) (v) vary trip lengths.
Thanks everyone for your advice and suggestions - especially Nekhbet for the toy and bone advice, and Mosh whose approach we are going to try in the future once dog learns to focus on us and can listen even when he is excited.