Many people with borderline personality disorder also have PTSD, but it also seems to be common for BPD to be misdiagnosed when only (untreated) PTSD is present.

Can anyone point me at research identifying differences which would indicate whether BPD is present alongside PTSD, or whether PTSD only is present?

One that comes to mind is age at onset: BPD is normally evidence by late teen years, symptoms do not just appear later in life. PTSD can be delayed onset so some time after the trauma occurs (sometimes, years).

Links to research would be really helpful, I have the DSM-IV and DSM-V criteria already.