Okay, so I have figured this out now. I was using 'Line Sequence Number' to identify the line of the transaction to be updated. It turns out that as hinted at by Ely Gulen, what I actually needed was the 'Line ID'. If you look at these two fields they may give very similar results, but they are not the same thing. I could…

Sorry, I hadn't even realised that the VAT Periods could be locked separately, which is where I was getting in a muddle. I was expecting the report to lock the periods automatically. I have now been corrected, and am back on track. Thank you for your response.