|RI connection cable|
Ah yes, there is, but none of it directed to my question
Onkyo's RI connection is described on some websites as a proprietary Onkyo connection, hence the question here.
However, during the course of the day, Onkyo UK emailed me to say that there's nothing special about the jack or the cable. I suppose the RI system is proprietary, rather than the cabling.
Anyway, depending on what my ebay seller does next, I may get to put this to the test...