Hi Gary,
I was having the same problem with my S Type - 2.7 diesel, and the AA indicated that there was one EGR Valve that was using the problem.
I got my mechanic [top man] to look at it and while he ordered a new EGR valve he had misgivings but felt that it could be somewhere in the exhaust causing the problem. The exhaust is a stainless steel one.
My mechanic had a friend who was an expert in cleaning exhausts who took my car and ran a system through the exhaust that cleared out anything that was causing the problem. The cost was £180, and the performer was just was if the car was brand new..
Try to find something like that in Lancashire.
Regards,
Peter.