Sophos Firewall API - Creating User

Hi, looking for some help regarding User creation on the Sophos Firewall using the API.

We've managed to be able to update a user's group membership, but are unable to actually create an account using a similar XML format.

Currently we're getting the following error:

    <Login>

        <status>Authentication Successful</status>

    </Login>

    <User transactionid"">

        <Status code"510">Operation failed. Deleting entity referred by another entity.</Status>

    </User>

 

The XML is:
<Set operation="add">
<User transactionid="">
<Username>UserX</Username>
<Name>Userx</Name>
<Password>Password</Password>
<UserType>User</UserType>
<EmailList>
<EmailID>UserX@gmail.com</EmailID>
</EmailList>
<Group>Guest Group</Group>
<SurfingQuotaPolicy>Unlimited Internet Access</SurfingQuotaPolicy>
<AccessTimePolicy>Allowed all the time</AccessTimePolicy>
<DataTransferPolicy/>
<QoSPolicy/>
<SSLVPNPolicy>No Policy Applied</SSLVPNPolicy>
<SSLVPNIPv4Address/>
<SSLVPNIPv6Address/>
<ClientlessPolicy>No Policy Applied</ClientlessPolicy>
<L2TP>Disable</L2TP>
<PPTP>Disable</PPTP>
<CISCO>Disable</CISCO>
<QuarantineDigest>Disable</QuarantineDigest>
<MACBinding>Disable</MACBinding>
<MACAddressList>
<MACAddress></MACAddress>
</MACAddressList>
<LoginRestriction>UserGroupNode</LoginRestriction>
<ScheduleForApplianceAccess>All The Time</ScheduleForApplianceAccess>
<LoginRestrictionForAppliance/>
<IsEncryptCert>Disable</IsEncryptCert>
<SimultaneousLoginsGlobal>Enable</SimultaneousLoginsGlobal>
</User>
</Set>

Any help greatly appreciated as we're running out of ideas