Thank you burakkucat, and the interesting thoughts kitz. The I dragged the offending mail from the wastebin, which I should have deleted, and checked the headers again.
What I did find curious is this
'Return-Path: <bounce-1123346-27699000@email.get-pdfsuite.com>'
at the very top.
Now I have had spam from get-pdfsuite on a number of occasions, but not addressed the the named email account.
The logon name that was used belongs to the laptop that occasionally use which has never been used to send an email from.
I am therefore looking closer to home and wondering if there is some kind of virus on one of my Linux machines.
Time to have a look at clamav I think. The rootkit checks have not found anything. A real mystery.