Hey there! I have no idea what the contact info for Martha's is. However, I bet you can run a search on any Belize website and find it. It's right in the middle of town (and the action!). I have no idea how late you can book a reservation on tours, but I know we booked them "last minute." Eva's is probably the best place to go to inquire about tours.
Have a great time! I know you'll love it!