Ah yeah, that's for the OP.
My problem seems fixed. After trying twenty random different things it was either removing a map.ini file that I still had in the data folder (no idea why?) or changing the height of the spawn empties all to 2m above the ground.
I now also understand my issue was a bit different (rather cars falling to the side of the track, then rolling when I set the spawn objects to 0m); when the car stopped rolling I could throttle but it wouldn't move.
Hope that helps for anyone having the same issue I had (so again, different to OP's issue).