@Guy-L 

If this is an old house, its electrical wiring might not be up to powerline demands. Worst case Deco units won't be able to use powerline, or it'll be extremely slow and unreliable. Yet, everything might work and you'll get good WiFi coverage all over the house. 

You would not know until you try, so I strongly, really strongly, recommend you get Deco P9 set from the place with good and easy return policy. One month return policy should be enough for you to figure if that solution works at your house.

What you are buying is WiFi mesh, under TP-Link Deco brand. With WiFi mesh the whole idea is all units work as one. You will not need to configure each unit individually with SSID/password, for example.

In your case, I suggest you configure your Deco to run in Access Point mode. By default, it runs in Router mode. Connect first unit to the router (BT Hub 5) via Ethernet cable. It will be your Main Deco. Configure it according to user guide. You will set SSID/password as one of steps. Then, follow this document: Switching a Deco to Access Point (AP) Mode

After that, you can add all remaining Deco units to your Deco mesh. They will all run in Access Point mode.

When you have set WiFi on Deco and can connect to it, it is recommended to turn off WiFi on BT Hub 5 router.

>then have 4 other units to locate in the 'dead spots' do I configure those in Access Point mode?

Bring each unit to where Main Deco is, and add that unit to Deco mesh, following user guide for adding Satellite Deco. Then, when it is successfully configured, take it to where 'dead spot' is. Power it on there, it should join Deco mesh and, if electrical wiring is good, will use powerline.

Repeat same with remaining units.

>I would want to connect a television using an Ethernet cable to one of the remote units 

This should work with Deco unit and is a good idea, as long as that unit uses powerline and its powerline link is stable.