In reply to captain paranoia:
Here's the email I received, 17/07 (I emailed them the same day):
We need you to log in.
We’re doing some essential maintenance work on our platform and as a registered user, we need you to take two minutes of your time to log in to your account. This will make sure your account is up to date.
Log in now (button)
If you don't log in, next time you do log in you’ll be asked to reset your password to access your account. If you have any problems, please take a look at our how to video which will give you all the help you'll need.
Here's their response to my feedback, received 06/08:
Thank you for getting in touch with us regarding the recent email you were sent about your OS Maps account. We understand you may be concerned and would like to reassure you that this is a genuine email from Ordnance Survey that we sent to you. You will also find messaging on our website on the OS Maps pages to confirm this.
We are upgrading OS Maps and improving our systems to help you enjoy the outdoors to its fullest. As part of this, we’re moving to a new system to manage accounts and we have asked all of our customers to log in to refresh their accounts.
When you log in it will update our new system with your up-to-date information and ensure that you maintain access when we turn off the old system.
We hope this answers any questions you had, and we encourage you to follow the guidance in the email.
Post edited at 12:17