@Kevin_Z Hi - thanks for your thoughts on this. I've tried a hard reset (to no avail), and - on checking with the TP Link support chat - the conclusion is ... it's broken ! Unforunately it is out of...

My TP Link AC 1750 ADSL2+ modem seems to have developed a fault whereby the power light is on (and remains on constantly), but every few seconds the wifi light will come on then go off; shortly...