Many thanks for the replies.

It does not matter what I'm trying to control (fwiw a combo of Denon DVD player, receiver, and humax PVR using RF to the RM433 then IR out to devices), the behaviour is consistent and the fault seems to lie with the RP6:

It's like this: the remote seems to be doing fine, as the RM433 is flashing pretty heavily. The problem is that that RP6 is lazily sending only one command and takes about a second to do so.

SDZD, I presume by "sustain" you mean "repeat while button is presssed"? I do not see mention of any property with the specific name "sustain".

I have a TV being controlled in standalone mode, and I'm pretty sure that it can do that without delay. I'll double check when I go to site today.

So nobody's had this kind of quirky behaviour?