All good thanks.
I realised that the snippet was coming from %excerpt% in the Title&Meta settings.
I then read up about how to implement my own custom codes and have implemented one based on your informative and helpful articles (and a bit of AI support !). This works just fine.
One thing – I note the advice is to edit rank_math.php but that does not seem a good idea as it will be overwritten on the next update. I implemented mine in the child theme functions.php file and it works – so is thee any reason not to do that ?