As Evan said, rent a car from Leverick. Best way to see the baths plus you get to see much more of Virgin Gorda. The views alone are worth the rental.

But what would even be better is if you re-arranged the order of things to be at Leverick the night of the 20th. As I've said in other topics, two nights at Leverick is ideal. You could simply skip a night at Cooper or Peter to fit it in. We try to get in by 3 or 4 on a Thursday. Get a rental car and go out to dinner at Coco Maya (Think we'll try Chez Bamboo next time). Then Friday we often (when we have friends who haven't seen it)go to the Baths for breakfast and to see the baths. Then we can go see the Copper mine, Hog Heaven..etc. Then get back to Leverick in the afternoon/evening and hit the BBQ around 7pm. Depending on what time you get back to Leverick, you can also fit in a visit to Saba Rock for some drinks and the tarpon feeding.

If you went around clockwise, you could probably fit Leverick in at the end of the week and spend your last night at Norman. We've often left Leverick on a Saturday morning and been at Norman for the night.