Until very recently the UDP section of the “Core Code (Firmware)” documentation at http://docs.spark.io/firmware/#communication-udp contained a useful link to the community thread which most usefully described issues and workaraounds to the still broken UDP functionality. There is now advice to find details of the necessary workarounds and the limitations of the Spark Core UDP implementation in the “Known Issues” category here at http://community.spark.io/
The issue remains, and needs to be fixed, and if the text is mine that makes no difference. I suppose I’ll have to re-learn how to do changes to the docs.