@Richchad 

TP-Link also sells Deco X50 and X55 in 2-unit sets. If you need 4 units, buy two 2-unit sets.

I would recommend X50 or X55, they have better feature set than X20.

All Deco models will work with powerline adapters, as long as adapters are compatible with Deco Ethernet backhaul protocol. If your adapters are not, you may need to replace them with compatible model. TP-Link brand is recommended, but I tried from ZyXEL and they worked also.

The easiest way to test compatibility of existent pair of powerline adapters is to connect them to Deco units on both ends and see if Deco units will report Ethernet link over them.

When you get Deco mesh hardware and do initial setup, if you have questions about verifying powerline adapters compatibility I can provide more details. 

  @Alexandre. 

Thanks for your reply which is most helpful, the powerline adapter I have is a Netgear XAVB5004 bought about 12 years ago.. it works with my existing Tenda system so maybe okay with TP-Link Mesh system..?

I guess like most people all we want reliable WiFi and good coverage at the lowest cost, our existing WiFi is used mostly for phones, some smart plugs, EV charger and streaming Netflix etc..

  @Richchad 

I can't tell for sure if Netgear powerline adapter will be compatible with Deco mesh, but you can test it when you have Deco units. If it is 12 years old and not compatible, maybe it is time to replace it, too. 

I usually do not recommend Deco X20 model, unless someone has very tight budget. It is the budget model of WiFi6 Deco mesh lineup. Deco X50 and X55 are in the middle by performance and price.

Also, X50/X55 have better feature set than X20. You might not need that, but if you come to this forum later and say "my Smart Thermostat does not want to connect to Deco mesh," the last thing you want to hear back "this is addressed in firmware for X50/X55, do this and that and it should connect, but you have X20 with limited feature set firmware where it is still a problem without good solution."

What I recommend to everyone buying networking gear such as WiFi mesh is to definitely get it from the place with good and simple return policy. Deco mesh is not a complex system, in under 2 weeks you'll figure if it is working for you. With 4 weeks return policy you should be able to send it back if it does not meet your expectations.

  @Alexandre. 

Yes I might need to replace the Powerline, we shall see.

I normally buy this sort of stuff from Amazon and appreciate the 30 day returns option, unfortunately they don't seem to have X55 in a 2 pack at the moment but do have a single what they describe as Deco AX3000 addon unit which I assume will be compatible with either X50 or X55.

The X55 seems to be slightly cheaper than the X50 and looking at the specs I can see the only difference is the black top on the X55?

  @Richchad 

Check both X55 and X50 on Amazon. Where I am (Canada) Amazon currently only has 3-unit sets of X55, but 2-unit sets of X50. I'll put screenshot at the bottom of this post.

X55 and X50 are so close in hardware specs and feature set, it does not matter which one you choose. If both models can get you the right number of units, I suggest to just buy cheapest of these two. 

Scroll through Amazon, you might find 2-unit sets of X50 or X55. If you have absolutely no choice but buy 3-set X55/X50, the best possible add-on unit will be X50-POE. This one: https://www.tp-link.com/en/home-networking/deco/deco-x50-poe/v1%20(1-pack)/

X50, 2-unit set, Amazon Canada:

  @Alexandre. 

This is the one I thought was meant to go with the AX3000 range... but if you think the X50-POE you suggested would be better albeit £42 more expensive then I will go with that?
If I go for the X50-POE and the AX50 is there any benefit as to which I use as the primary connection to router?

  @Richchad 

TP-Link has huge variety of Deco models and their naming could be confusing. I can find at least three Deco models listed as AX3000: X50, X55 and X60.

Among these three, X50 and X55 have three Ethernet ports, X60 has two Ethernet ports. You don't want X60.

According to product description from screenshot, this AX3000 has three Ethernet ports. Which means, it should be either X50 or X55, most likely X55 as US TP-Link Deco Web site says X55 can be sold in single units.

Long story short, it appears to be the right add-on unit - you should get it while it is on sale, in addition to 3-unit set of X55s you mentioned earlier.

After you configure Deco mesh using Deco app, it'll report you actual Deco model (X50, X55 or what else), I'll tell you where to find that info.

  @Alexandre. 

Here is a screen shot of the same page posted earlier and it does show it as an X50, so I have ordered 3-unit X50s plus the single X50 as the X50s are on offer a bit cheaper than the X55s.

Thanks for all you help with this.