I inadvertently double-clicked in GMail and selected "always display images from firstname.lastname@example.org" This wasn't what I wanted but Googling "gmail browser list of senders you've allowed to display images" returned info on how to display images or not globally in the Settings menu, but not how to turn it off where the link within an email has been clicked.
I have found the way. Select the 'details' down-arrow to the right of the 'receiver email address' shown in the header. This will have an entry at the bottom of the dropdown to allow images to not be displayed.
I am noting this here in case it is helpful to others.