Below is some further information received direct from Meridian, re-printed here with their permission, as it may be useful for other HHs.

Originally Posted by Meridian
We've since noticed your post on Hitchhikers where you mention the elusive "500 Series User Guide". We're fairly confident that it was never created, despite the reference to it in the 504 guide!

When 500 series was first introduced, the use of the "Function" key provided access features or functions on products that were not the Controller product - the 504 or a CD player, for example. However, when we started making surround-sound products and DVD players, we had to adapt our comms system to deal with the menus carried on the disc that had to be navigated just to play a film.

One change we made was that we put Surround Controllers above DSP speakers in the "hierarchy" of which product becomes Controller when using the Clear key. In effect, we added a new "layer" of functionality which meant that some IR control options became unavailable even when using the Function key.

Not that it is a solution for you, but we have memories of having to temporarily disconnect comms leads to access display options on some product combinations.

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