Thanks @arminunruh you're a star. That literally did exactly what I needed, it just doesn't work on mobile though I think it needs the mobile title class? I tried doing this below but no luck:
jQuery('.sitetitle-txt-inner span' , '.mobile-title text is-fixed').text(word);
I've tried learning some programming over the years, but CSS is as far as it goes, and the good old "copy/paste/tweak until it works" approach. Appreciate your generous help.