Mrs.Dougall, the Chinese Embassy in Madrid doesn't accept visa application by post, you can submit visa application in person or entrust someone else (e.g. travel agency, visa agency)to submit visa application on your behalf.

You'd better tell me your itinerary, if you travel Shanghai, Yangtze River, Beijing, Hong Kong, Perth Australia, then Mainland China, you need a double-entry L visa because you enter Mainland China twice, if you go to Shanghai, Yangtze River, Hong Kong, Beijing, perth Australia, then return to Mainland China, you need a multiple-entry visa because you enter Mainland China for 3 times, go to Hong Kong means exit Mainland China, they apply different exit & enter controls. British passport holders are exempt from a visa to Hong Kong, so you don't need a visa to Hong Kong. If you still have questions, please post your enquiries again.