Like minds.... Been looking at a similar itinerary, London - Phnom Penh - Siam reap - Phnom Penh - Sihanoukville.

Plan is 2 nights P.P, Fly to S.R 3 nights 4 days, Rtn flight to P.P, Taxi to Sihanoukville,

Cannot seem to get direct to Siam reap as first stop in Cambodia, connections with massive transit times and costs excessive and it seems to be cheaper to pre book London to P.P than to try to do independently, Flight to be P.P via Thai or Singapore at the moment are £1200+ per couple (you can do cheaper but involves an extra stop)
P.P to Siam Reap Bangkok Airways about 5 flights a day approx £220 per couple Return. You can do this journey via coach or fast boat (google fast boat, long trip and looks a bit seat of the pants!)
The taxi guides below give a good itinerary to use even if you don't use there services

P.P Hotels from £20 - £85 looking at The Pavillion Hotel Clickyfor character or Quay Hotel boutique Clicky
Siam Reap Day Inn Ankgor Resort Clicky

Taxi/tours P.P Clicky
Taxi/tours Siam Reap Clicky

Some of the Hotels are much cheaper through Agoda Clicky

Welcome your research and Ideas to, hoping to go March

Mids

When we go to Cambodia we alway's fly into Bangkok, then use Air Asia to Siem Reap or Phnom Penh. If you went to Siem Reap first you could alway's get on the bus to Phnom Penh which take's about 5 hour's and cost about 12 dollar's. We are off to Phnom Penh and Sihanoukville again in November we have 2 night's in Bangkok 1 at the start and 1 at the end, 2 in PP and then coach to Sihanoukville for 11 night's.

Was finding Bangkok air very expensive so would probably do Singapore air with short transit in Singapore then onto Siem Reap, bus/boat to Phnom Penh, car to Sihanoukville with flight out of Phnom Penh at the end- was working out cheaper than going through Bangkok.
Will check out Air Asia

Did Siem Reap 4 nights / PP straight through to Sihanoukville via Mekong Express = 7 nights /back to PP - 2 nights/Bangkok 1 night.

Look into getting a Discovery Pass from London branch of Bangkok Airways..minimum 3 flights ..maximum 5. Mekong Express EXCELLENT and SAFE! I emailed our hotel in Siem Reap (Bopha Angkor - EXCELLENT!) and they organised the return tickets for me. In Sihanoukville stayed in brilliant hotel..Reef Resort. Back in Phnom Penh we stayed in Billabong Hotel..again..EXCELLENT!

We travelled 3rd March this year. Feel free to PM me with any questions.

For a real taste of Cambodia I would highly recommend travelling to Siem Reap from Bangkok on coach, entering Cambodia at Poipet and travelling the rest of the way (to Siem Reap) on a minibus. This sure beats the easy flight into Siem Reap option which so many tourists take these days and therefore miss out on one of the great overland trips in the world.
It's not for the feint hearted, but it remains one of my highlights from our RTW trip of 2006.
The contrast in infrastructure is fascinating from Thailand to Cambodia. At one point you are tearing down well maintained tarmac roads with service stations, and then after Poipet it's dirt tracks, potholes and broken bridges.

It's 10 hours total journey time from Bangkok to Siem Reap but it goes so quickly. Grab a bite to eat the Thailand side of Poipet, then walk the no mans land between both countries before entering Cambodia.

The rewards of starting your trip to Cambodia in this way will live with you for the rest of your life....and I don't mean the permanent bump on your head from the Cambodian mini bus journey!!!!

We did the Siam Reap - Phnom Penh - Sihanoukville route.

Our trip was to KL with a one night stop in the airport Pan Pacific...then a morning Malaysian airlines hop to the Borei Angkor, Siam Reap after a good sleep.
You're spoiled for hotels but we wanted something good value in the town and not stuck out on the highway towards Angkor Wat,....some very expensive hotels there but hopeless for seeing how the locals live. Hotel was fine...great pool and breakfasts...tuk tuks outside to take you anywhere. Best food we had was at the FCC...nice bit of history and good cooking.
One thing we loved (after Indian shambles) was the Cambodian online VISA. Apply at home online, attach a .jpg photo and details....VISA is emailed then you print and take with you. A quick barcode scan and retina photo at Siam Reap and you're into baggage and outside to the pavement bar for an ice cold beer. For such a poor country they could teach Goa a trick or two.....

4 nights there...then we took the coach to Phnom Penh...stayed at the gorgeous little "Pavilion" (google it...), then took a taxi to Sihanoukville for 7 nights.
Taxi back to Phnom Penh and an Air Asia hop back to KL for 3 nights.

We went via KL because the flights seemed better value than Bangkok, and we love the atmosphere in KL....very smart around Petronas and the park, but within 10 mins of Bukit Bintang and street food. Stayed at Traders with a Tower View room.....amazing view....people have been known to just sit there at night staring out of the window! Well recommended and not overpriced....great Sky bar on the roof which also houses the swimming pool.

The timings worked out fine and we had no problems with the trip. Loved Siam Reap and Phnom Penh.....though might be tempted to go to a Thai beach rather than do Sihanoukville again.