Provide the last four digits of the account number you downloaded the CSV from and upload the full statement (not the mini-statement) for your current account, savings, isa or credit card.
if you would prefer to run this on your own server the project is now hosted on GitHub. Feel free to download, fork, maybe even add some documentation.
if you are not comfortable with this, don't use the service.
The privacy concerns above will be the same for *any* conversion service or application. I assure you that I have no interest whatsoever in looking at other peoples finances, I am not an accountant and never want to be.
If something hideous happens, you can find me as @edwilde on Twitter or leave a comment here so others can see.
This converter is in no way associated with Nationwide Building Society.