Thanks for the responses.
I have checked the hoses and resealed the screen/filter connection with a new gasket.
I think it is a pump problem as the pump is showing signs of oil leaking, not a lot but it is leaking.
The oil I use is the same as the oil in the 12V71n's, a straight 40w recommended and supplied by the local DD/MTU dealer.
The oil and oil filter was changed about 50 hours ago. I think it is time to try another change.