Sounds fine.
My POF funds were in a bank which couldn't provide a letter either and there were no issues - 6 months of stamped statements, generic letter stating how long I had my account and that I'm in good standing and I wrote an explanation in LoE.
I was also added to the account literally a couple of days before the application (officially the account was in my wife's name but in reality we both had access to it) and that didn't raise any questions either.
Don't worry, it sounds like you've provided plenty of information