Problem connecting Apple TV to Airplay Speaker

Hi

I am having problems trying to get my 3rd Gen Apple TV (1469) to send audio to an Airplay endpoint.

Here is the scenario I am trying to achieve. We have the Apple TV located near a projector for use with iPads to screen share. Unfortunately there is no audio near the projector so I was trying to then get the Apple TV to Airplay the audio to something else near the PA desk so we can get audio when we are showing videos in presentations etc. I have an airport express that we use for the wifi and in the past have had no issues in connecting the 2, however this seems to have stopped working.

Here's what I have tried

- Everything has been updated to the latest software levels (7.2.2 on the Apple TV and latest firmware on the airport express)

- I can Airplay to the Apple TV fine and when I have it plugged into a TV we get audio no problems.

- I can Airplay audio to the airport express from my phone with no problems

- When I go into the Airplay menu of the Apple TV and try to select the airport express as a 'Speaker' the audio briefly drops then just returns to the TV. No sound ever makes it to the airport express.

- I have also tried using a second Apple TV I have as an airplay endpoint and the same happened

- I also tried the same experiments with the second Apple TV and got exactly the same results

- I have retried everything with and without wifi, with and without an full internet connection with the same results

I know I could use the optical out, convert it to analogue and run a cable down as the optical out is working but I am trying to avoid the cable run as I think what I am trying should work.

Are there any known bugs or gotchas when trying to connect a 3rd gen Apple TV to an Airplay speaker please?

Thanks for your help

Chris