The deal says that vouchers cannot be combined to extend stay, so you can get a 3, 4, or 5 night stay.

Hopefully Gail will confirm and/or let you know what additional nights may cost (may vary according to high or low season). I like to go for at least 10 nights and like to spend some nights on the cliffs, so 5 nights works fine for me. That means that I'll be able to take advantage of the included transfer one way only, but that's still good.