May contain traces of nut
Any further ideas before I go and bang my head on another wall...................???
The key (possibly) is that if your OK button is knackered (many are), the handset might not register a continuous press. When you hold OK continuously, does the PVR button light continuously, or only flash? It needs to light continuously. If it is only flashing, you need to repair the handset contacts - see the HD/HDR-FOX wiki HERE (click). (The HDR-FOX does not respond to a long press on OK, only short presses.)
Once the handset passes the test, try to get it into channel change mode in another room (away from the Humax). This is what works: press "0" then "OK"*, holding both down together until the PVR button lights up continuously (the PVR button lights up briefly when "0" is pressed, but then extinguishes when "OK" is pressed until it lights up again to signal entry into channel change mode). Press EXIT to cancel.
* The instructions published in post 1 say "0" and "OK" together, but "0" slightly before "OK" works better than "0" slightly after "OK".
When you have rehearsed the process, trying it in view of the Humax with only one box powered up (disconnect the other from the mains) will produce an on-screen message "you can change the remote control mode...", also telling you what the current mode (control channel) is. Release the handset buttons, then press one of "1" to "6" to select a new channel. Changing the control channel will dedicate the handset to that particular Humax box (or at least any Humax boxes operating on the same - new - channel number).
I have uploaded a video of this process so you can see how it works (if a picture is worth a thousand words, a video must be worth millions):
NB: When viewed on my Windows PC, this video has captions. When viewed on my iPad, the captions are missing.