Dihydrocodeine is not hydrocodone. It is a narcotic analgesic, but that's all I know. I'm not a pharmacist, and I don't know anything about the chemistry. There isn't much information available in the US since dihydrocodeine is seldom used here.
From what I have read, it's not as effective as hydrocodone for pain relief. But dihydrocodeine is used extensively in Europe and other parts of the world for pain and cough suppression like we use hydrocodone or codeine.
Do an internet search for Co-Dydramol or dihydrocodeine, and I'm sure you'll find a lot of information, both chemical and practical.
Here is a link to the product characteristics - maybe this link will answer your questions better than the patient leaflet: