@Alexandre. 

Thanks, that was an excellent explanation of how both cell phones and mesh wifi works.  Does it mean that the wifi firmware in my phone is able to talk to the mesh system and arrange an orderly changeover in the same was that the cellular firmware in my phone can talk to the cellular system and arrange an orderly changeover?  I have always assumed that the idea of mesh wifi is recent and that the cell phone would not 'know' about the concept.

  @Alexandre. 

I did reply thanking you for this but it seems to have disappeared, so thank you, this was a very good explanation of how a mesh system works and how it differs from an ad hoc selection of access points.

Going back to the original problem, I have now carried out another couple of experiments.

The first was to link the main deco to a satellite with a network cable.  The 'ethernet backhaul' icon appeared on the deco APP between the two, so I think that the powerline icon should appear if it was working (it doesn't appear)

I then moved the main deco another 15 meters away from the garage deco so that it is now a long way away from it.  It no longer connects to the garage deco.  This to me means that the powerline backhaul is definitely not working and that it was relying on a very weak wifi signal.  As I mentioned in an earlier post, I have used other powerline devices on these electrical sockets and they have worked fine.

It' definitely going back!  I will report later on whether or not the replacement works. (should be arriving today)

  @AllenK 

I've sent the first one back and ordered another one.  It's just the same so I thought that I would install the firmware update as recomended by David:

For the second issue, it is suggested to have a look at this link to check whether P9 is on powerline backhaul and also test whether the beta firmware here could improve the current issue a little bit:

Improve Powerline Backhaul on Deco P9

Unfortunately that didn't work either so given my experience and that of various others who have bought these products I'm beginning to wonder if any of them actually work at all over the powerline and that a lot of people just have it working over WIFI without realising.  I noted that the installation instructions say that the decos should be within 100 feet of each other, which is probably the maximum distance that the WIFI works over - I can't see any other reason for having them within a certain distance if the powline feature actually worked. 

I had previously connected the main and a satellite with an ethernet cable and I noted that with the old firmware the Deco APP displayed a backhaul icon with the word 'Ethernet' next to it whereas with the new firmware and an ethernet cable it just shows the backhaul icon with no indication of how backhaul is achieved.

I also paid more attention to the 'seamless switchover' from one deco to the next.  The animated advert shows somebody walking around the house and the switchover occuring almost as soon as the person goes past the half way point.  This is certainly not my experience.  I've used a WIFI monitor to see the relative signal strengths as I move from the connected Deco to another Deco.  This also shows when the switchover occurs.  The experience that I've had is that I can be within a few inches of the new deco and it is still stuck on the old Deco, switching after a considerable time (20 seconds onwards).  So it does eventually switch, but not until some time after I've got very close to the new Deco.

In my situation I have the main Deco in the centre of the house (which is where I am most of the time so I'm connected to it most of the time) and one on each end of the house which are there to handle the garage at one end and the electric gate controls at the other.  Neither of these walks around the house (I don't live in a Steven King movie luckily) so not having the backhaul or rapid switchover is not the end of the world and my main reason for getting the system was to have a single SSID everywhere, which is now what I've got.  So I'm keeping the system, but it's an expensive way of doing it and I'm not happy!!!

  @AllenK 

Hey, 

Just got my P9s today and I have exactly the same problem as you. Powerline does not seem to work at all. Firmware update says they are up to date. 

7 year old Av500 mini Powerline Adapters work without issues when being placed in the same sockets. 

I am also opting for returning the P9s and get another set. Looking forward to read about your replacement.