We have a hotel booked in SF and then want to visit, Carmel, Monterey, Cambria, Big Sur (sorry if these are in the wrong order - am from the UK) which will be 14th - 25th September and hopefully Jackson and Murphy. As we won't know how long we will be stopping at each, will it be a problem getting accommodation as and when we stop or is it imperative we book ahead?

I don't think any of these four places have a problem with overbooking - especially since you'll be in the US a little past "vacation season". Most of us will be back in school or back at our jobs.

I would only pre-book for major cities (such as SF) since business conventions, festivals and other special events can cause hotel rooms to dry up. I'm not aware of any festivals, etc. during Sept. 14th-25th in the places you listed.

Don't pre-book anything accept for the final day where you are departing from. You may not stick to your US itinerary anyway. If you pre-book you will be stuck with a schedule you might not like.

I assume that even if you don;t pre-book you will still do all the hotel reseach in advance - so you know where you'll want to stay and can call to reserve the night before - or at least in the AM - rather than starting to look for random places in 5 inthe afternoon - unless you're willing to take anyplace.

Most of those areas will be fine w/o pre-booking. But I would definitely consider booking for the weekends. Even though the main tourist season ends on Labor Day (5 Sept this year), Friday and Saturday nights will still be busy in places like Big Sur/Carmel.