Hi Duncan,
If you’d like to host your website with the new hosted service, please refer to the link below for detailed steps:
onlinehelp.microsoft.com/.../hh335299.aspx
It mentions that the A record should be added before you point the domain name servers to Office 365. Therefore, please change the name server records back to point to the domain provider and then add the A record.
When you change your domain’s name server records, all the services that are currently associated with your domain are affected. You can create Office 365 email addresses with both the tenant domain (such as user@domain.onmicrosoft.com) and your custom domain for your users so that our users won’t lose email messages when you update the domain name server records.
In addition, after you adding the A record and changing name server records to Office 365, do not change the website URL back with your custom domain. Once you done that, Office 365 will redirect the website to the domain.sharepoint.com again.
Thanks,
Nancy Xie