From UK to china

I am from the UK, i want to visit China in the summer time. we went to the travel agency and they said the visa is only available for two years and it is impossible to get it for a few months and that it cost £270 per person is that true? Can we get a visa ourselves without the travel agency and how much would it cost for 2 adults, one 15 year old and a two year old for a 3 month one if it is available and if not how much is it for two years. And approximately how long does it take to get it done ?