I considered an RF repeater but discarded this option when I discovered that they can cause interference with wireless Ethernet networks.

I have partially answered my own question by finding some long range 'blaster' emitters from Audioplex. However, Audioplex tech support has made similar comments to those above from Hoop. They say it is important that the equipment receives a signal from only one source at a time. You also need to avoid the emitters being 'seen' by the receiver I would point my remote at, otherwise you will get feedback. In my situation, this would only be possible if I were to put my kit and the emitters in a closed cabinet.

Let me ask a different question. Has anyone got any experience of the passive 'IRPrism' devices that simply stick on top of the IR sensor of each AV component? These are claimed to increase the angle of acceptance to 90 degrees from the central axis - and might therefore be a very simple solution to my problem.