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 are not comfortable with this, don't use the service and complain to Nationwide - they are the ones who have told their customers to put trust in a third party to put their financial data into OFX format.
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.
I would still prefer that Nationwide brought back their direct OFX service, so please keep the complaints flowing. Send a secure email, complain over the phone, write a letter to head office, attend a talkback session or complain to the FSA.
Follow the progress at betteronlinebanking.co.uk.
This converter is in no way associated with Nationwide Bank or Wesabe.