Take a look at the method described here: Need layout solution for nested portals...
It uses a "detail" portal to display records related to the selected portal row--which is more than you need, but the same method can be adapted such that you just put a large multi-line text field next to the portal in place of that second portal so long as you add it for the table occurrence that you would otherwise have used for the "detail" portal.
Thanks for the help. I had tried something similar to the advice given in the link you supplied (based on some google searches) but I think I got the details wrong as it didn't work as I'd hoped.
After doing what your link suggested things working as I wished (almost) however I made two changes:
First, instead of placing a button in the portal rows to call the script I used the portals "Set Script Triggers..." OnObjectEnter event to call the script. After doing this selecting a portal row did cause the associated comment field to change according to the portal row selection however the field selected within the row would not activate, i.e., couldn't be edited. So I also removed the "Commit Records/Requests..." command from the script called by the OnObjectEnter event. The field entered in the selected portal row is now active (editable).
Do the above changes seem reasonable or is there something else I need to do?
If you need to edit the fields, the changes you describe seem reasonable.
Thanks again for your help.