@jamieman5 

Basics first: Deco mesh can run in two distinct modes, Router and Access Point. Some of answers on your questions will depend on how you plan to deploy Deco units (wired, wireless) and what mode Deco mesh will be running.

If the reason you are buying Deco is to improve WiFi signal in your house, you can run Deco mesh in Access Point mode, which allows maximum flexibility in how Deco units can be deployed. See this TP-Link article for How to set up your Deco in Access Point mode.

Assuming your Deco mesh runs in Access Point mode:

firstly i will hardwire my ISP modem/router to deco box one, will i still be able to use the remaining 3 ports on the ISP box + 1 remaining on deco for physically attached devices,  or will these now be deactivated when deco is attached?

 

You will be able to use the remaining 3 ports on ISP box and 1 remaining on Deco. Your ISP box will treat Deco just like another wired device connected to it.

The only thing you will need to do with your ISP box is disable its WiFi networks.

also does deco box 1 have to be cabled to ISP or can it connect wireless?

 

Deco box 1 (Main Deco in TP-Link terminology) have to be wired to ISP with Ethernet cable. It is a requirement.

Yet, Main Deco does not have to be side by side with ISP box. For example, in my house ISP box is in the basement at the corner. My Main Deco is located in the middle of the house on the first floor, wired to ISP box with Ethernet cable and gigabit switch in between them. 

You said there are wired devices connected to your ISP box. I can't tell from your post where these devices are, but if one of them located in place that is better fit for Deco node, you can connect Deco to Ethernet cable there and connect wired device to the second port of that Deco.

If this place happened to be in the middle of your house, this is where Main Deco may go.