Below is a explanatory letter I wrote to the travel agent I booked the flight with. They were extremele good, which in a way proofs that my complaint was legitimate. They refunded to me the amount they made out of the deal, partially compensating me for my losses.

The airline concerned was China Southern Airlines, flights CZ 306,307,308 and CZ 335.
On my return flight from Amsterdam to Auckland I could get only the first boarding pass from Amsterdam to Guangzhou and was told, the second boarding pass would be waiting for me in Guangzhou. That boarding pass was handed to me by the pass control at the airport in Guangzhou.
However, the seat was number R44D, a seat in economy class, while I had booked and paid for Premium Economy. I questioned the allocation and the stewardess referred me to the head stewardess who referred me to an administrator from outside the plane.
Unfortunately the command of English language of the administrator was very limited, so he was either unable or unwilling to give an explanation why I was not offered the seat I had paid for. The plane certainly had a premium economy section. All he was explaining was, that it is not his fault and I need to complain to the ticket issuer in Amsterdam. I wasn't concerned whose fault it was. I only wanted my premium economy seat with more leg room as I am 66 years old and have severe leg problems in the small economy class seats. The unpleasant conversation ended in an ultimatum by the administrator to me, either to leave the aircraft or to accept that seat on offer. I accepted the seat under protest.

Could you please be so kind to pass on my letter to China Southern Airlines with the request to reimburse the difference between Premium economy I paid for and the economy seat I was offered.

What makes the whole issue particular galling is the fact, that I changed my initial booking with China Southern Airlines. I had overlooked that the flight via Beijing was economy only for the last flight, Beijing to Auckland. Given my leg problems I paid the penalty for changing the flight to Guangzhou to ensure I would fly all the way premium economy only to end up in a situation I desperately tried to avoid.

While writing on my experience with China Southern Airlines, I would like to bring to the attention of the company other deficiencies.
I had requested Asian Vegetarian meals for my flights, but was offered a vegetarian meal only on the first flight. The other three flights I was offered meat dishes and only after pointing out the fact, I had booked Asian Vegetarian, I was offered an alternative, usually "vegan"which I don't like but had to accept in the circumstances.

There were also problems with the onboard entertainment systems. Only on the first flight everything worked. On the three remaining flights I wasn't able to operate the touch screens and on the advise of the stewardesses I used the controls in the armrest. However, even with the assistance of the stewardesses I was not able to watch any movie.

All in all, I must say, a very disappointing first experience with this airline