Interestingly just had a letter through from HSBC US via my virtual mailbox after updating my US mailing address details -
There's then a 7-page form that has to be completed!

Unhelpfully then then suggest replying by post although I'm going to try emailing them to see if there's an online submission route! Has anyone else had this?
PS addendum - looking into it in more detail was actually quite easy to reply (e.g., their online payment verification system can't handle non-US number formats) and I suspect it may have been me opening a savings account that pays interest that did it!