Can you actually fly between these cities - I have given up trying to find an operators website ?

Yes;

Cambodia Angkor Air This is there face book page, CAA

for some reason their website is down, www.cambodiaangkorair.com

The website currently is under the construction for online booking and web check in. It will be available for passengers to access flight information soon.

If you would like to view flight schedule and make a reservation, you can login to www.vietnamairlines.com. Noted that for sector of PNH to REP, view flight and booking could not be process. Please call to +855 23 212564 for booking and reservation.

plus soon u will be able to fly SR to Shinoukville as well

http://www.phnompenhpost.com/index.php/2011083151357/Business/sihanoukville-flight-claims.html

There are 3 direct flights scheduled every day from REP to PNH and vice versa.

First flight leaves REP 0840, then 1130 and 1720.

Today 18 Sep for instance the 0840 was cancelled but both the 1130 and 1720 departed pretty much on time, actually both arrived PNH early.

You used to be able to book on the Vietnam Airlines website but are unable to now for some reason.

I'd suggest ringing Vietnam Airlines.

The Cambodia Angkor airlines website is back up now, but only links to VN Air for booking.

However u can book online thru the PTM Tours and Travel site mentioned in the Cambodia Angkor site but its coming up $175us o/w. Fare should be around $100us including all taxes.

There are 3 direct flights scheduled every day from REP to PNH and vice versa.

First flight leaves REP 0840, then 1130 and 1720.

Today 18 Sep for instance the 0840 was cancelled but both the 1130 and 1720 departed pretty much on time, actually both arrived PNH early.

You used to be able to book on the Vietnam Airlines website but are unable to now for some reason.

I'd suggest ringing Vietnam Airlines.

The Cambodia Angkor airlines website is back up now, but only links to VN Air for booking.

However u can book online thru the PTM Tours and Travel site mentioned in the Cambodia Angkor site but its coming up $175us o/w. Fare should be around $100us including all taxes.

From their facebook page,

Cambodia Angkor AIR

We would like to inform that Big Promo is still valid until 15 Oct. Please hurry up to get your low fare now.

$175 o/w doesn't sound like to much of a promo for a 45 min flight