Finally worked out what the issue with upgrading out machines to Windows 10 from Windows 7 was – if you get a 0xC1900202 error this is “0x80240020 WU_E_NO_INTERACTIVE_USER Operation did not complete because there is no logged-on interactive user.” This points to this Microsoft Article which explains the issue can be resolved by a registry key.
Locate the registry key: [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\OSUpgrade]
It should exist, but if not, create it.
Create a new DWORD (32-bit) Value with Name = “AllowOSUpgrade” (without the quotes), and set the Value = 0x00000001.
Check for updates and Windows 10 should now show as avalible.
This does not appear to exist as a GPO setting, so you will need to put this reg key in with your WSUS GPO (or a similar GPO policy)