I'd send a letter recorded delivery with the unopened letter from them attached. State date of purchase and the lack of a forwarding address. Last thing you want is a judgement against the address that you'll never know about and then coming home to changed locks.
thanks folks - had the meters reset (they were pay as yo go) and since changed to a smart meter, so that side of things was sorted way back when.
will drop the debt collectors an email with a burner account, as i'd rather they didn't have my number.