Right, problem solved. I think.

Greg is right about skyscanner, I used it to find the same flight and booked through opodo.co.uk. No passport number needed.

The only time (apart from in China) I've needed to enter a passport number for booking flights was on flights to the USA. You don't have to provide a passport number at the time of booking, although you are encouraged to; however, you must provide your address details (if you are a non-citizen and the USA is your final destination) and passport details etc. at the latest during check-in. What bugs me about this is that the USA doesn't require such details from its own citizens. Call me crazy but that seems like the kind of racism or discrimination or whatever you want to call it that the USA claims it does not tolerate...remember those clauses you always read about: "...does not discriminate on the basis of national origin, etc. etc." but in this case the USA does, by not requiring that information from their own citizens. So much for equality. Anyway, I digress.

Chinese immigration is generally fairly friendly and anything is better than US immigration anyway. Yesterday at Kunming airport a Thai citizen was talking on his phone in front of passport control and was politely told to stop, but didn't. After that no further attempt was made to stop him, yet in America you might be denied entry or taken in for questioning if you did that.

FYI, if a US visa remains valid in an expired passport, those lazy bastards at the US embassy/consulate won't even bother transferring your old visa to your new passport, but instead you'll be required to travel with both your old and new passports and show both at check-in at the airport and US customs/immigration upon landing. I'm not sure how China does it, but I'd imagine it would be transferred to your new passport. Best advice is to ask if this is possible, but I know many countries that would transfer an existing visa from an expired passport to a new one, including Thailand.