When and where should i apply for the visa

Am i able to get a chinese visa from somewhere else in Asia such as bangkok because i am going travelling for 3 and a half months before I actually reach china so if I applied for a Visa in England it would have expired by the time I actually reach China! Do we need a letter of invitation or just hotel and flight confirmation?
We are flying from hanoi to Xian then travellig accross to beijing, then flying home. We will only be in China for 12 days.