Just a quick update on this.
I contacted the BT Helpdesk again yesterday afternoon and chased this one. They are advising that the details have been sent to the UII ASG team for investigation. (ASG is the BT Application Support Group, UII is the BT Uniform Inventory Database).
We suspect that the Inventory database may be holding incorrect port information for some circuits and this is why the Performance tests fail to work correctly.
Should have an update by the end of this week, but depends on how long it takes for ASG to get back to the Helpdesk.