Placed it within a tab - my understanding is that a tab behaves no differently to a section but I will test it with the field out of the tab just in case. Do you know whether that might make a difference?
In the meantime I have set up a workaround using an Action to create a new timestamp record in another table each time the field is edited and then added a summary field to the original table which shows MAX date created from the timestamp table. A little more messy but with the added benefit of being able to eventually graph user engagement over time. During this exercise I have discovered the limit of 10 Actions per table since I originally began creating one Action per field that I wanted to track. There are 20 in total worth tracking, but thankfully only 4 are multi-select so I currently have 4 using Actions and the remainder of the fields are tracked using dynamic form rules. The reason for having one Action per field is because I need the Action to pass on the identity of the field that has been edited. Currently doing this using "write the value". Also, the user is likely to edit more than one field each time, each one requiring a new entry on the tracking table and I'm not aware of a single Action being able to create multiple records at once. Any ideas to the contrary would be really helpful!
In summary I now have timestamps for all 20 fields (4 of them multi-select), thank you for your help.