I would suggest making sure that Harvey has had a really good walk beforehand, the kind that would normally see him heading for bed as soon as he gets home, so that he has less energy to play up with. You could also practice leaving him in the car for short times (say 5 mins) and then gradually build it up, maybe with a stuffed kong to keep him occupied.
For all my practising for the Bronze I used treats, but occassionally would only pretend to have a treat and the dog would follow quite happily thinking that I had a treat (its mean I know). Sometimes I would make sure that I had treats in my pocket and make sure that my hands smelt of them even though I had nothing in my hand, and it seemed to be enough to keep the dogs attention.
Also, when we did the test it was also indoors, but we were the only ones in the room - examiner, dog trainer and me and the dog, so there weren't any distractions from other dogs/owners which makes it a lot easier when you're doing your sit/stay.