Thanks again for the tip Jane which got us seats at the front. When we set off the bus was full but it then made several other stops picking up locals (without livestock) who picked up a stool on entry and sat in the aisle. The noise was pretty deafening with everyone shouting either to someone further back the bus or into their mobile phone.
For the most part the roads varied from poor to nonexistent. We did have a rather "interesting"" experience when the driver, despite intensive use of the horn, slammed on his breaks and was forced to pull on to the verge to avoid an errant motor cyclist. We bounced along for what seemed like a few minutes to the accompaniment of shouts from the locals sitting beside the driver and complaints about the suspension from those at the rear.
En route we also discovered the secrets of Cambodian pig transport but I'll leave that hanging.
We had a 15 minute stop half way and arrived safely in Phnom Penh after 6 hours and about 300 kilometres. 350 if you allow for all the swerving to avoid potholes.
Now in our spacious, clean if somewhat minimalistic room. Have just skyped Jenny which was lovely. Now shower then dinner.