@Red_Nation is the Deco part of the broadband package that Sky provided or is it your own network equipment? My first thought would be that if it's very close to your new Sky router you'll need to disable the wifi on it to prevent interference with the Deco wifi. You might also need to set the Sky router into modem-only mode to disable the router functionality, or if you prefer to keep that set your Deco mesh into AP mode. You could also try enabling/disabling QoS in the Deco app, if your connection is now faster and you have old QoS limits in place you'll be throttled. I've also heard reports of QoS fixing local connection speeds when the limit is set appropriately for your connection but no idea why!

  @Red_Nation if you have an FTTP connection and a standard ethernet cable between the "box in the wall" and the Sky router you might be able to remove it completely and just plug the Deco straight into the FTTP modem, but a quick Google seems to suggest that the M4 isn't compatible with Sky's FTTP configuration.

For enabling AP mode in your current setup, the following link should walk you through the process: How to set up Deco to work in Access Point mode | TP-Link United Kingdom

Provided you disable the wifi on the Sky router and swap the Decos to AP mode it should hopefully improve your connection speeds