Hi @Retired_mod ,  it does feel like something to do with a limitation somewhere but the confusions with that are 1: It has worked fine for months with much more data (I did limit the table to 500 records after the issue manifested itself, 400,000 in original)  and 2: other people in the team with essentially the same setup are no suffering the same issue. 
We were wondering if there was something subtle in the config that is causing this.