Thanks AJ, loads of info there! We went to Wave Rock today and actually had the same idea as Tina (great minds think alike haha!), took a picnic and decided we'd stop in York on the way home. However by the time we got to York we thought we just may as well keep going until we got back to Perth. Thanks for all info anyway.
Just as an aside, we did the trip in one day and both thought it was well worth the drive. Had a great day out with lots of stops for photos along the way so the drive didn't seem too long. I know most people here would recommend not doing it in one day (and I wouldn't recommend it if I were a tourist only here for a couple of weeks) so it's good to have another perspective.
Depends on the type of person you are I suppose, my husband enjoys driving and didn't mind doing the long journey and I was glad we didn't stay the night, as there would have been nothing more to see the next day that we hadn't seen on the way without going further on (to Esperance etc).