1.9 dCi oil pressure warning at low revs

My Koleos has recently developed this problem. When engine warm the low oil pressure warning signal comes on. A slight tap on the accelerator (to rev over 1000 rpm) and the signal stops but repeats some seconds later. Service is not due (unless previous owner arranged something....)Oil is clean - I topped it up ( it was only a little below full).
How should I source the cause of this? Could it be
a. faulty sensor
b blocked filter
c. oil pump
or something else.

1.9 dCi oil pressure warning at low revs