Hi Vj,
The sender system is SAP, and the idoc appears to have been sent from there correctly.
I have reimported the idoc structures into PI as I noticed that the one IDOC segment had been enhanced and this hasn't been updated accordingly in PI. Somebody has told me that if the segment mappings don't match exactly then the IDOC essentially disappears i.e. it appears as sent from the sender, and in PI it simply isn't there!
I have requested another IDOC to be sent to see if the change above makes a difference. Will keep you posted.
Thank you once again for your help.