but am a little confused by how a subwoofer really helps in a car. (...) At home, definitely - it fills in the sound gap at the bottom end, and just makes the sound bigger (...)

Exactly. The same things that make subwoofers sound good in home stereo systems is what makes them sound good in cars.

but in the car road noise covers that frequency range, so you need to turn it up too high to make a difference and then it just hurts.

Because most car speaker systems simply can't reproduce that frequency range at all, it's not just a question of compensating for road noise. It's about getting the music to sound right, with or without road noise.

If you're in a car where "it just hurts", then the system simply isn't tuned well.

I would like to be convinced that a subwoofer will be worth the loss of space in the boot, but experience has shown me otherwise. So far.

I understand what you're saying, and I know the kind of experience you've had. It's too bad you're in Scotland, otherwise I'd demonstrate my system for you...