Hi Graham,
Sounds like the pump timing is all out of kilter.
More than likely because of a failed needle lift sensor on No.3 Injector.
Disconnect it with the engine running and see if there is a difference in engine note.
Meanwhile, do a few searches around the forum for topics relating to the needle lift sensor. You'll find plenty of test procedures and similar symptoms.
EDIT:
Try
this one
And
this one for starters...
Don't worry they refer to a Citroen. It's exactly the same on yours...