Hi Dave
Thanks so much, I totally should have seen that!
I think what confused me is that the certificate customising help page (https://docs.moodle.org/30/en/Certificate_customizing) mentions x,y distances, image sizes in pixels, and 'lines' (without specifying what those are either) as in: "Say we want to move it to the left to the 100 position from 130 and up to the 435 line from the 440 line."
It would perhaps be good if this could be edited by someone to point this out more clearly.
Thanks again!
Nicola