Reset modem & router, XET1001s only work same room

As per title: we've had a pair of XET1001's in the standard config for a while, at least 3/4 of a year, no problems. I just came home (this is in my parents' house) and rebooted the modem & router while detangling wires in the hope of moving the router somewhere higher to get better reception throughout the house. Wifi working fine afterwords*.

BUT: the desktop PC in my dad's office that's the only thing running off the 'receiver' XET1001 isn't connecting.
Central blue light on both send & receive XET1001 blinking at a regular rate, one fast blink every second or so.
Tried everything to diagnose - IP6 turned off, rebooting computer, etc etc..

Moved receiver XET1001 to an outlet 1 metre from the sender: works.
Elsewhere in same room: works.
Anywhere else in the house: doesn't.
I've tried to manually add the sender using the utility, while plugged into the receiver in the office, but I'm not sure how to add it: I was expecting 'name' to be MAC address and password either to be the hardcoded one or HomePlug, but none of those combos work.

I've also tried plugging both XET1001s into the same power strip, but since they already work in the same room this is kinda redundant. When the receiver is in the office the lights look right, but I can't get a connection. When the receiver is in the same room, it just connects automatically.

Any thoughts or ideas gratefully received. Kinda amazed that this seemingly completely illogical situation has arisen, sucking three hours of stepwise diagnostics out of my evening, with no return.

Cheers gents!

*the signal WAS slightly stronger, I know you were dying to find out.