To follow on from the above note, my new longer C5 cable is now plugged in and the 504 continues not to respond to the display key! General operational functionality is absolutely fine.

I have just removed the C5 from the 504 and tried the remote display without success. The display button on the 504 works fine so I shall settle for that!!

Cinema: DSP750 x3, DSP3300 x 4, DSW.2 x 3, Trinnov A32, 271, Oppo 203, Toshiba HD-DVD, SKY Puck, Sony XW7000 PJ, Screen Research AT, Kaleidescape 4K 30TB
Lounge: 861v8, 808v6, UHD722, DSP8000SEPP, DSP7200HCSE, DSP3300, DSW x 2, 504, C15, Sony UBP-X800, SKY Puck
Bedroom: DSP5200SEHC, G65, UHD722, G08, DSW, M6, C15, 210, SKY Puck
Spare: 818v3