Thanks Swiflet. Here's a summary of other information from Lamma mums about their delivery experiences:
re ambulance
Call 999 to get the ambulance if you can't walk to the clinic or not confident to go on ferry.
How long the ambulance takes to get to you depends on where you live and whether the ambulance guys know how to get there. I kept the police map with me. It shows the lamppost numbers which the cops use as references.
http://www.compunicate.com/Lamma-zine/Blog/Blog-2005-10.html#Oct5
re helicopter
Suggest you just get to clinic a.s.a.p. then doc will check how far you are and decide if you need to go ferry or helicopter.
if everything's suddenly happening fast
If you feel the baby might be coming very soon, tell them about that when you call 999 to order the ambulance. (Tell them it is quick and you know you will need a heli.) There is no way to book a helicopter, only the 999 and clinic route.
If you're not happy with the uncertainty then we'd suggest staying in town close to your chosen hospital for piece of mind.