Hey Alistair, your read-only link isn’t working but here’s how I’d approach the situation:
Highlight the text you want the underline to be on then wrap a span around that text > Give that span a name.
Now click on the class and add a background image > select the image and use the controls to make the image span the area and align to the bottom. [You may also need to increase line-height of your text]
Let me know how you get on + please reshare your read-only link! 