[Conkerman]

Do you have any old routers kicking about?

A very quick google seems to suggest there are issues with airplay and certain routers.

I'll have a loom one the sprogs have been put to bed.

To add.
If you have an old router connect a computer to it and go into the menus and disable DHCP. This turns the router into a network switch.

Replace the EE router with the 'switch' and see if that works any better. I would seriously think about moving everything so you can discount an issue with the powerline adaptors.

There were definitely a few disgruntled people having drop outs with Airplay and routers seemed to be a factor.

Hope this helps, I may not have an answer straight away, but we can eliminate things one at a time!