1st Q: Yes it is available, but only if the router/SkyQ hubs have the correct firmware to allow you to switch it on. Mine came live about 6 weeks ago.
2nd Q: It is compatible, but SKY have set their own password so your power line devices can not talk to the SKY powerline devices. They operate in parallel. So you actually have 2 parallel powerline services working in your home. I have used this same config for 6 weeks and have not noticed any difference.
3rd q: the way I am reading this is the SKY powerline network is a "private" network and for now you can not use your own or a possible SKY powerline device (which is not available or being discussed at all on the boards). The SKY powerline devices are connected inside the router/box devices and for now are just to allow them to talk to each other to make the internet functions of SKYQ work.

Firstly: Sky haven't officially announced that the powrline function has been released despite the return of the configuration option on the hub.

Second: I do have it enabled and I have noticed a slight drop in speed on my AV 500 network

Third: The powerline "kit" is built into the Q hub, Q box and minis so no external powerline plugs are required. The Q powerline network is set up as a private network and, as Batboy says, is an integral part of the AirTies system that connects the hub and boxes and as such can not be used by non-Sky devices.

Thanks to all who replied and thank you for the information regarding the powerlines/homeplug compatibilities.

With regard to WPS; the Sky Q Installation guy said that WPS had to be kept switched on, (all of the time, even after all the boxes are set up and linked wirelessly), does anyone know if WPS always has to be switched on or can WPS be switched off ?

when I switched it off, the SKYQ mini could no longer connect to the SKYQ silver box. When I switched WPS back on (I do not use it) the service burst into life.

In reply to a post by jb66:

Apart from the fact you'll be lucky to get more than 60mb/s

With the powerline OFF all seemed to be OK and via wireless I was getting a fairly steady 37 Mbps.

I switched the powerline ON, (via the router which resulted in a reboot), and the wireless speed dropped by over 30% to less than 24 Mbps.

I decided to switch the powerline back OFF, (via the router), but no reboot occurred and the wireless speed stayed around 24Mbps.

I pushed the reset button, (on the rear of the router), and it came back at 37 Mbps.

I have no idea if the powerline is worse or is better but the wireless speed did seem to be much worse when the powerline is switched on.

Regards,
Fido

That's not uncommon power line is sometimes slower and drops out (like in my home).

5Ghz WiFi is pretty good especially when you have a decent signal.

Stick to what works best for you.