Rather surprised that there was no feedback to this question. This forum seems very quiet. Both from general users and, especially, from TP-Link tech support (seeing this is an official TP forum).

So I did a bit more digging around myself and finally found this - http://www.tp-link.us/faq-440.html

Have set this up and now my Archer C2 is acting as a Wireless bridging Range Extender to my C8, both for the 2.4 and 5GHz bands.

Note - there seems to be conflicting advice concerning use of both bands. One Statement says:
" Note: Here we use 2.4GHz wireless network as an example, if you want to WDS bridging the 5GHz wireless network, please go to Wireless 5GHz accordingly."
Elsewhere it states -
"WDS bridging only requires the WDS setting on either the root router or the extended router, on either 2.4GHz or 5GHz; NO need to setup on both side or both band."
bands. In fact it is worth emphasising that you MUST only set up one router and one Frequency band for the system to work in ALL modes. I set up the C2 in the 2.4 GHz parameters to have WDS Bridging ON - that's all. If you also set the 5GHz band with WDS Bridging on it will not work.

Also you need to manually set the bridged router to use the same channel number that it finds from the Survey/Scan operation in each band. For some reason it doesn't do this automatically.

This works well and my phone and tablet can now connect the 5MHz band all over the house. Which makes movie streaming, either from stored files or the Internet much more reliable to wireless devices.

The only issue is that, at the moment it has different Network Names for the Upstairs and Downstairs networks. If I simply rename the two pairs of networks to the same names will it then seamlessly switch to the router with the strongest signal wherever in the house I am. Selecting 2G or 5G connection first?