Thanks for taking the time to check it. I have now gone back through every setting and found the problem. I had the Method in the MP3 Encoding dialogue set to “Highest Quality”. I now know that this method will not produce 48KHz mp3s. Changing this setting to “Fastest” solves the problem.
Although there is normally a warning message when using that combination of settings, there is a peculiarity whereby the warning is not always displayed.
If you set the sample rate to 48KHz then set the method to Highest Quality, Wavelab warns you that the sample rate will become 44.1KHz.
However, if you set the sample rate to 44.1KHz, then set the method to Highest Quality, then go back and change the sample rate to 48KHz, then no message is displayed (unless you go back into the MP3 encoding dialogue).
Although this might be an unusual sequence of events, I think it would be useful to be presented with a warning message at the time of changing the sample rate, or perhaps when actually rendering.