Thanks @Dave-TP I've decided to return the M9 Plus as I found that I couldn't use the Powerline backhaul on the P9s at all with the M9 as the main, and it didn't seem to be worth it to use them as...

Our home has very thick stone walls, and is relatively large. We find it difficult to get a decent signal throughout the house, even with 3x Deco P9s Today I bought 3x Deco M5 and 2x Deco M9 Plus....