@AdvLuke 

To run Deco mesh in Access Point mode, you must be running Sky Router in its default gateway/router mode. Which means, all Ethernet ports of Sky Router should be active.

We can use that.

The first suspect is always Internet (fiber) link. If you have Windows PC (desktop/laptop) you can hardwire directly to one of Sky Ethernet ports I can give you simple script that will run non-stop and ping well known Internet site. Script will only record when ping fails, which means day or night you'll see if/when Internet link went down.

I used this script to troubleshoot issues I had with my cable Internet: intermittent brief outages that were quite hard to track. After I established it were problematic link to Internet, the rest was easy.

I wrote that script for Windows, because this is what I have. 

  @Alexandre. I beleieve my Sky Router is in Modem mode, with the Deco's in Access Point (acting as the "Router") with the Sky Router WiFi broadcasting disabled

  @AdvLuke 

TP-Link uses the following terminology for Deco Operation Mode:

1. Router - default mode, Main Deco can be connected to either ISP modem or gateway/router;

2. Access Point - Main Deco must be connected to ISP gateway/router.

What’s the difference between Access Point mode and Router mode on the Deco?

Using consistent terminology would simplify troubleshooting. 

If you run Sky router in modem mode, there is no simple way to check Internet link, unless you want to disconnect all your household from the Internet. My recommendation would be to switch Sky router to its default gateway/router mode and then you can use free Ethernet port on Sky Router to run health tests for Internet link, without disturbing household Internet access.

  @Alexandre. Thanks again for your reply

I can't reference where I have this information, as it was 2 years ago now when I set it up however, I do recall that Sky Broadband does not like users using Mesh (or any 3rd party router system) so putting it into modem mode was the only way forward 

Although I am happy to revisit this based on your information & will look into it further! 
 

Do you know of any solid websites / posts with advice on setting up Mesh WiFi with Sky Broadband ISP settings etc?

  @AdvLuke 

Not sure why that would be: for ISP router Main Deco should appear as just another client, such as PC. 

No special changes should be necessary if Main Deco configured with getting dynamic IP address.

Just configure Sky as router and if Main Deco can't work, undo it and make Sky run as modem again.