I have had the exact problem myself, it's dead easy to fix. There my be upto 2 oil pressure switches/sensors. One on the cylinderhead, and proberbly one somewhere on the housing that the oil filter screws on to. Disconnect the cables going to them. Unbolt them from the head/housing, and clean them out with some sort of aerosol solvent (Brake cleaner is ideal) The old oil/gunge will come out of the inside and bobs your uncle. Do not get them mixed up as one is a low pressure sensor qand the other high. This should fix the problem. Please3 email me if the problem persists
turbodiesel.head@ntlworld.com All the best David