I'd like to recommend
www.airchunk.com, with over 30 groups and 400 users using the system already and excellent feedback from them all.
It's secure, free and super easy to use.
Oh and there's a UK telephone number to call if you need help and emails answered asap!
Thank you to all the users who've provided excellent ideas on new features like a sunrise and sunset time. More great ideas in development as we speak.
If you'd like a booking system that suits you, try
www.airchunk.com
Enjoy,
Abe