Hi.
Having read and thought about both of your suggestions, here is the workaround I've developed that seems to work:
– In each image, through Bridge or when creating them in Photoshop, I insert the description in File Info;
– I create a paragraph style in Indesign, 'Advert';
– I place the images into my document and then follow these instructions using this script: https://creativepro.com/modified-script-adds-captions-from-clipboard-to-images; I select XMP Description from the label type, and then Advert from the label style. If you have multiple images to apply captions to, leave them all unselected and the following notice displays 'Add captions to all images in the document?'. Click OK and job done;
– Once the captions (tags) are in place (automatically created on their own new layer called 'labels') I uncheck the Print Layer box under the Layer Options for 'layers';
– I then create a TOC using David's suggestion above, making the list alphabetical.
This seems to work well, though I can't group an image with it's caption as the image won't then print (I imagine the no print option for labels is applied to the image). I suppose a work around for this is to have the label text have no colour instead of the label layer Print Layer option being unchecked, but the downside is that you don't get to see the label then.
Anyway, I hope this helps and if anyone has any other suggestions I'm all ears.
Cheers.