Business trip visa application in Phnom Penh

I am a UK national living in Phnom Penh. I hold a full UK passport with a current business (E) 1 year visa for Cambodia. My company is a UK based consultancy firm. We now have clients in China and I need a multiple entry 6 month visa to visit the clients. Is it possible to obtain this in Phnom Penh?

If so, do I need any further supporting documents in addition to the application forms e.g. a letter of invitation from the client and/or a letter confirming the client work from my company and/or anything else?