I noticed the system light is continually flashing - all the others stay fairly solid (though the wlan flickers a bit)

Read LED explanation on page 3 and 4 of its user guide. If there are some lights abnormal, try to reset it. If it won't help, it should be faulty.
http://www.tp-link.com/resources/document/TL-WA730RE_V2_User_Guide_1910010693.pdf

Thanks for your help. Re-reading the manual I realised the flashing system light is, for some reason, a sign that the device is working normally! Oops!

However, I still can't log onto the device. The manual is a bit vague on specifics sometimes. Am I right in thinking that after a hard reset (with the pinpush button at the back) I am supposed to plug the device into the ethernet port of my computer and go to the address 192.168.0.254 ? I've also tried doing the same with it plugged into the router, and also looked for it showing up as a new wifi network. There are two other IP addresses written on the top in very faded pen - 192.168.1.255 and 192.168.1.1 so I also tried accessing them with no luck. Any ideas?