I was the controller who heard the garbled message fragment. I can say with some confidence that it did not come from a "United" aircraft [after thorough investigation of the RT tape replay]....however there was an aircraft which may have been the United flight referred to in some reports that did descend from FL370 to FL310 to start the APU....and which may have subsequently descended to FL250 before leaving UK airspace. Both of these occurrences happened in the same 10 minute time frame during the very early hours.
The response time was outstanding.....well done the boys in blue..awesome!