how to tell which sophos central a endpoint is connected to ?

Hello Simon Wilks ,

You can disable tamper protection using this method if you cannot locate the device, and then re-install Sophos with an installer downloaded from the correct Sophos Central account.

Otherwise there is C:\ProgramData\Sophos\Management Communications System\Endpoint\Persist\customeridentifier.txt that may help identify which Central account the device is in. The content of the file is not an explicit name, so you will need Support's help in tracking the account name down.

Thanks,

In Central, the URL for a computer takes the form:

https://central.sophos.com/manage/endpoint/devices/computers/41b17f17-cd53-04a6-bb5c-ada356253ee7

so:

https://central.sophos.com/manage/endpoint/devices/computers/

plus the unique ID for the computer.  This ID for the computer can be found in:
\ProgramData\Sophos\Management Communications System\Endpoint\Persist\EndpointIdentity.txt

If you're logged out of Central and visit the URL, for example:
https://central.sophos.com/manage/endpoint/devices/computers/41b17f17-cd53-04a6-bb5c-ada356253ee7
you will be asked to log in and then it will redirect you.

If you choose the right login, you will get the summary of the device, if not you will not.  This might save a little time.

Otherwise, from either the contentes of CustomerID under: HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Sophos\AutoUpdate\ or from C:\ProgramData\Sophos\Management Communications System\Endpoint\Persist\CustomerIdentifier.txt you can get the customer ID, the machine belongs to.  

If you're in the Partner Portal, if you open the Dev Tools of the browser, and view the page that has all the accounts, in the API response (XHR) you can find the JSON that has all the customer IDs, this also has the friendly name of the customer if that makes sense.

Another option might be to add a "marker" that is unique for the customer,  E.g. In global scanning exclusions - https://central.sophos.com/manage/config/settings/scanning-exclusions you could create a simple marker for each account you manage e.g. a Windows Exclusion for "AccountX".  You can then check the client under:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Sophos Endpoint Defense\Scanning\Config\ OnAccessExcludeFilePaths for the value.

Regards,
Jak