Answering my own question above.. this DIDN'T work.
The TV seems to only send signals down the cable connected to IR OUT for devices it has been set up to talk to via the Universal Remote setup process.
An 3.5mm male-to-male IR lead going from the TV to the receiver worked (Samsung remote could power on and off the amp) but only after it had been set up in the Universal Remote setup. The Denon amp remote did not work at all before or after the setup, nor did any other remote (Sky HD, Matrix, etc). In other words - the TV does not passively rebroadcast all IR signals it receives down the IR OUT cable, only the ones from its own remote and then only when it has been explicitly programmed to via Universal Remote setup.
I'm a bit stuck now - anyone have any ideas on what my options would be? I guess some kind of single IR receiver that distributes that to several outputs?