@EvilDoctorK 

Another test: bring both the 'Garden' node and TL-SG1008D into the house, connect TL-SG1008D to Zyxel switch, connect "Garden' node to TL-SG1008D. If Ethernet backhaul is stable, the problem is not with switches but in the 'grey' link between them.

  @Alexandre.  - Yes thanks .. good suggestion I will add that to the list of tests to try and will report back !

Further testing 

- Looking at the TP-Link switch in the new building .. the Green activity light on the LAN port that takes the network feed from the new Cat6 Cable faceplate connected to the house doesn't show continual green, only intermittent brief flickering   ..  If I take the TP-Link switch and bring it into the house and plug it straight into the switch then it gives a solid green light on the LAN port that I plug in straight away 

- When I then plug the 'Garden' Deco node in to the TP-Link switch that's now directly connected to the main switch it also produces a solid green light on the switch LAN port that it's connected to and it settles onto the Ethernet backhaul after a few minutes and seems stable there 

So this is working then as I want it to .. though only in the house which is obviously no use ! 

( Sidebar note - the lag on the Deco app updating can be a bit frustrating .. e.g. it will show nodes disconnected even though they're showing the white lights .. so not sure I entirely trust it all the time .. it's very slow to refresh the network map showing ethernet vs Wifi node config - killing the app and restarting it seems sometimes to produced a quicker update  ) 

So this seems to indicate potentially an issue with the new cabling / faceplates  ... any ideas what it might be worth looking for ? .. the odd thing is it has worked at least once so it's not as if it's totally not connected on the cable which is why I was thinking the something to do with switch incompatibility / network loop prevetion etc. etc. 

Just to confuse things though if i totally bypass the switch in the house and just connect the LAN port on the main Deco straight down the new Cat6 cable to the new switch/node in the Garden building it works fine and holds the Ethernet backhaul no problem 

So if it can do this happily and work it can't be a cabling problem then really .. it has to be something else ?

( though if I remove the TP link switch at the end and just connect the Deco through the wall plates to the main node it doesn't work ! )

  @EvilDoctorK 

Yes, that gets confusing.

Bring back Zyxel switch to its original place, unplug everything from it but the link to TL-SG1008D. Also, unplug every device from patch panel connected to Zyxel.

If you have stable Ethernet link as reported by TL-SG1008D and Satellite Deco stays on Ethernet backhaul, plug back one by one devices connected to Zyxel until issue reappears. If it does, you have found the troublemaker and if it does not - you've got working network layout.

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I already participated in troubleshooting of two very interesting cases:

1. A child plugged both ends of Ethernet cable to same switch, somehow only Satellite Deco got totally confused;

2. Two Ethernet cables were used to connect one switch to another switch. Unplugging one of them cleared an issue.

I wonder if yours will end up to be the third case of such interesting type.

  @Alexandre.  .. Great thanks for the advice .. I will try that later and see if that helps and report back ! 

  @Alexandre. 

I tried your suggestion ( and a few other related variations ) 

One switch in the Garden , one in the house:

- TP Link switch in Garden connected through new Cat6 cabling to Xyzel switch in house .. everything else removed from the Xyzel   - No LAN activity light on either Switch

- Xyzel switch down to the Garden connected through new cable to TP-Link switch moved to house ( only connection ) - No LAN Activity light on either Switch

Both switches in the House:

- TP Link switch in House connected directly to Xyzel switch in House via a single short cable  - LAN Activity light shows on relevant ports on both switches .. reversing the order of connection same result - Activity light shows so it works fine

Only TP-Link Switch in the House 

- Removed Xyzel switch altogether and TP-Link switch in house with only one connection to the Cat6 cable to the new Garden building where the Deco 'Garden' Node is plugged into the wallplate directly  ... Doesn't work, Deco Node will not keep a connection .. flashing red light , briefly alternating to steady white .. Deco Apps shows 'offline'   ( so it looks like it's trying to use the Ethernet, but failing - not reverting to Wifi anyway ? ) 

So it seems the two switches are not capable of negotiating a connection through the new Cat6 cable by the looks of it ..  The work fine together in the house, but once you put the long cable down the garden ( and the two wallplates ) between them they won't hold a connection at all 

However the cable is working at some level since if there's no switch at all involved ( scenario I posted earlier with the cable to the garden plugged straight into the Deco ) then it works ... Curiouser and curiouser ....

I think this one is going to be a cable fault 

After the testing above I also invested in a cheap cable checker ( can be gotten for a few £ on Amazon ) .. When I test the cable run from the new  building to the house I get only 7 of the 8 lights illuminating on the cable checker .. so there's a fault somewhere on one of the strands in the cable  .. hopefully it's in one of the terminals rather than along the cable somewhere, but let's see ..  That would seem to explain the problem

Not entirely sure why the configuration directly connecting to the Deco worked, as that was definitely okay .. but obviously having only 7/8 of the cable working is going to be a problem and almost certainly will fix the issue I was seeing !