paddy,
I've seen one UM in Club Europe from Nice (LHR - NCE - LHR is almost a commuter flight for me these days): I believe it was transiting on to the Far East in Club World. But I agree, it's not usual.
Just as having 2 pax assigned the same seat isn't usual......last time it happened to me, I kept the seat even though I was second one there, because they'd got the special meal booked for that seat!