Dear Cliff,
Thank you for your continued support with this issue- I am very grateful for your time and expertise.
I have followed your instructions carefully and implemented the changes you proposed for the third step in the flow. The flow now runs to completion, but it doesn't populate the newly created single line text column with the values. I tried this on two different lists (including the test list where your original method worked), but no values are populated for either the test list or my 'real' list.
Here is a copy of the flow- if you see any error or have any other suggestions that I can try, I am grateful for any feedback/advice.
Please note: in the Compose step, the Inputs entry appears different after the flow is saved/run, but I have hovered over this to show that the expressions is still: items ('Apply_to_each')?['Custodial_Group/Value'])



Here is the results (or rather, lack of) back in the List:

Thank you again for all of your help and I hope that you have a nice day too!
Kind regards,
Nic