This site doesn't seem to be terribly mobile friendly, which is the way I imagine most owner drivers would be using it. I thought of a couple of things that could make it better:
1. Auto login - it's a pain having to log in every time, can't you set a cookie to remember user/pass like most sites?
2. Posting empty looks like a bit of a faff, all you really want to do is post the postcode of town you'll be empty in, when that will be, and where you're going back to, and type of van. Job done - I don't get why you would need to tick an "empty" box when the form is already for posting "empty", and all the other gubbins that is on the form currently.
Maybe if things like this were easier to use, more people would make use of it.