I was very disappointed that no one ever responded to this post.  The unit appeared to settle down for a while but it is now doing the same after I loaded the latest firmware.  I can see a number of similar threads regarding C6 dropping the internet with thousands of views so this is clearly a common problem.  I have noticed that there is never a solution posted by tp-link so it is clearly a problem they know about and are ignoring.

Having swapped out various items over the last few months I am now fairly convinced that the problem lies with the routing software, and that for some reason the internal router gets locked up, and the unit has to be re-booted.

I only bought the C6 because my main internet connection has been provided by a W9970 which has been absolutely rock solid.  It never gets locked up and never has any issues.  As others have commented, it looks like tp-link have lost the plot on router software. 

@Kevin_Z 

Thanks for the response

To simplify testing I have moved the C6 closer to the W9970 and connected the C6 WAN port to one of the LAN ports on the W9970 so there is no other equipment involved. So this is different to the situation I had previously where I was connecting using a LAN port on the C6 rather than the WAN, but in the past I changed to a LAN connection because of this problem - the wifi (mainly 2.4G) disappearing.  After a few hours the C6 has already started misbehaving.  The main problem is the 2.4G WiFi keeps disappearing, and when it reappears I can't connect to it.  The 5G is still available and when I connect on that I still have internet access. 

1. The Gigabit switch no longer relevant but in the past it was working fine and anything wired through the switch or by WiFi to the W9970 worked fine, the problem was only wireless devices connected through the C6

2. At the moment I haven't lost the internet connection only the 2.4G wifi, so I will do more testing tomorrow

3. I have made up and tested a new cat 5e cable for this new setup and the WAN light is showing green, but I do have a connection so that is correct.

4. As stated above I have reconfigured the setup so that the C6 is directly wired up to the W9970 and within a few hours the 2.4G Wifi has stopped working.  My previous experience is that the C6 will need to be rebooted to get the 2.4G working again but I will leave it like this overnight to see if it recovers by the morning.

My next test tomorrow will be connecting a C6 LAN port, rater than WAN, to the W9970

@Ian_M 

The C6 stopped working briefly on Sunday.  I was unable to connect to the 2.4G wifi and when I connected to the 5G the client reported "no internet" (the device had a bad IP address - I assume this was because it had lost connection to the DHCP server in the W9970).  Turning the wifi client off and back on cleared the problem, the C6 did not need to be rebooted and none of the lights or status on the web interface changed.

The configuration is still C6 WAN port connected to the W9970 LAN but following this last problem I set the 2.4G to Ch 11 and 20MHz bandwidth.