SOLVED! Thank you @tuanphan for providing the solution!
Having tested on my page it works perfectly - this CSS adds the subtitle without affecting pagination, browser tab text etc. and does nor require any SEO adjustment as with the previous <span> method. Very clean solution. For anyone unclear, the href is the page url, i.e. if work/ceramics
a.grid-item[href*="/ceramics"] h3:after {
I have also included formatting code with each subtitle to set the subtitle style, though there may be a simpler way to achieve this? Example as follows:
a.grid-item[href*="/url"] h3:after {
content: "sub 1";
display: block;
font-size: 12px;
line-height: 1.5em
}
Good luck all! Now to finally finish/publish that website...