I just made my own adapter cable to convert my Bose a21 XLR (XLR to GA) as some of our Airbusses actually use GA plugs
Anyway, I also had some difficulty to find the 5,23mm jack, as it is used only in the military/aviation. My local electronics shop had even never heared of it. I found it at Conrad electronics. As you are from the UK this should be the correct link:
Neutrik NP3CM-B,STEREO NP 3 CM-B Jack Plug 5.23 MM Straight, MIL-Type Number of pins=3/stereo Content: 1 pc(s) from Conrad Electronic UK
Enjoy!