I used WPS to establish the connection between my router and the extender.
This seemed to work, and I got three steady lights on the extender (power, connected and 2.4G). My devices (a laptop and an android phone) are both showing the _EXT and _5GEXT networks available, so, apart from the 5G indicator on the extender not lighting up, everything looked good ...
The password on the extender should have been the same as the router password, but instead it seems to have set itself to a random string of 64 hexadecimal characters (is this a 32-byte UID ?). The password on the router is unchanged)
Also, the router claims to be dual-band 2.4 / 5 G, but the extender only seems to recognise the 2.4G band. I followed the instructions and repeated steps 1 and 2 of the WPS setup, but this still only lit up the 2.4G light on the extender. As mentioned above, my devices are showing the _EXT and _5GEXT networks.
The router was supplied by SSE when they installed a fibre connection for me. According to the label on the back, it'sm model is 'techicolor DWA0120'