@Mattjtemp 

I might be able to help you with troubleshooting, but first I have questions for you:

1. I have M5s, I have never seen white light on it. Are you sure you listed the right Deco model? White light is also not on the list of M5 LED colors in TP-Link FAQ:  What do the different LED colors meaning of Deco?

2. What is your modem brand name and model number? I'd like to check its specs and description from the Internet.

3. If you can Ethernet to modem, does it mean you can use any of modem Ethernet ports for Internet access?

It will also help me if you can make screenshot of Update Deco page in Deco app and share it here. Example of that page from my Deco app:

Thanks for the reply. 

This is the main hub, (I recently moved it to remove any possible other interference but it is still ethernet connected and working), but every now and then dropping out, this is the white light it shows (the satellites go red until they reconnect). It could be said to be a yellow light, in which case it's 'starting up' - that reset was why I originally swapped the main hub in case it was a hardware thing - but something external must be triggering that restart.

This is the main hub info from the app, showing it's an M5

This is the Update Deco screen shot

Modem brand - this is a - Huawei DG804W-2.T5 - it came with the ISP who are Talktalk

As per the other Ethernet ports on the modem  -yes - I have the main hub deco plugged into one, then the main hub deco second port connects to my switch (which is a TPLink TL-SG116) and the other ports work with a direct lan connection, that's how I ruled out my ISP or the modem as the cause.

Note  -this configuration has worked for months, possibly years, and was only in the last 2 weeks an issue.

One other factor I was wondering about was Sonos - I have a number of Sonos speakers, both old and new, none are hard wired into the network, and I have seen issues recently but thought that was the new app update plus aggravated by the house internet issues.

However, I disconnected all Sonos speakers from the power today and saw the reset outage within 30 minutes of doing it, so I don't think it's Sonos either.

  @Mattjtemp I'm no expert but have you tried the M5 outside the holder, or even elevated slightly? I find my M5's get very hot, I actually made a little stand for them for ventilation purposes. Wonder if that could be having an impact on the performance for you

  @Mattjtemp just a suggestion.

you could also log in at http://tplinkdeco.net/ and view the logs, not sure it will help

You do have M5s, that white LED color you've seen should have been yellow. 

It was a good suggestion to check if Main Deco overheats. That could have explained reboots.

Deco logs are not very helpful in troubleshooting, also they got reset after each reboot. You will not find in them what triggered reboot. 

Before I offer more drastic troubleshooting steps, a question to make sure you haven't missed simple one. You said you swapped Deco units: but did you swap their power bars, too? If not, try that, and if that does not help - let me know and I'll give you next steps. 

  @ConcernedUser It's a good shout and I've saved them, will do so again when it happens and note the time, but I can't make out what they mean!

  @Alexandre. 

Yes, I have them in a socket mounted bracket so was a different deco and power adapter. 

  @Mattjtemp 

OK, so here is first recommendation: switch Deco mesh to Access Point mode. See Switching a Deco to Access Point (AP) Mode

You should be able to do that, because you have router Huawei DG804W-2.T5 in front of Main Deco. Huawei will be managing network, while Deco mesh will provide WiFi coverage.

In Access Point mode many Deco features will disappear, but this is the test to see if perhaps Main Deco can't handle the load when it runs in its default Router mode.

After that, there may be one more step to stabilize your Deco mesh, if switching to Access Point mode is not enough, but that next step requires Deco mesh to run in Access Point mode, so let's do that first.

  @Alexandre. 

Ok, I've switched to Access point mode, I notice that the Ethernet connection to upstairs no longer works, it only works on WiFi  - I presume that's because the network Ethernet connection runs through the main hub Deco and that if I connect my main switch directly to the modem then that will work again.

Either way, the WiFi is working and I have a continual ping running so will check in the morning if there's been any outages and will update. Thanks.