The seller said there was no paperwork. I have since removed front and back dust covers, and discovered it is the lower bass panel that is buzzing. Shining a torch upwards at that spot shows a wrinkled diaphragm. The seller says he is awaiting a reply from OTA. Tomorrow, if there is no news, I will ring them myself.
BTW, asking folk on diyAudio has elicited a mine of info. and help. It seems half the world is experienced in repairing and rebuilding these things!