Hi Glenn, thank you for your quick response. The very next online accounting software I looked at was called zoho and it has almost the same feature but if, in the same csv file, there are rows with the same invoice number then those rows get added as further lines of the same invoice. I'm guessing that information that is only needed once per invoice, like e.g. the date, is either ignored in follow-on lines or overwritten by the values given in the follow-on lines. Also they seem to have a more comfortable field mapping facility so you can use your own terminology in the csv file.
The downside of zoho is that I have to pay, but at least is does what I require. I suggest you take a look at it, there is a 14 day free trial period to do experiments like this.
Regards, Chris, UK