One single entry 30 day tourist visa cost $25, made through an online transaction.
The application is made through this site – http://www.mfaic.gov.kh/evisa
You will need to complete an online application form and upload a passport photo.
E-visas can only be used to cross certain borders into and out of Cambodia, so make sure you check the map on their website.
A note on getting a new British passport in Asia. Inconveniently you cannot get new passports at any of the British Embassies or Consulates anywhere in South-East Asia (and probably Asia… although we haven’t checked that far) anymore. You have to apply through the Hong Kong Passport Processing Centre. The whole process can take up to five weeks, upon receiving your application, so you won’t be able to leave which ever country you’re in until that time. The good news is that, although your old passport will be cancelled, you get to keep it with you. This means that you can still check into hotels, show the police etc… until you receive the new one. We’re going to get our new passports in either Australia or New Zealand.