Still no closer to solving this one. It turns out the pressure on the engine when cold is absolutely fine, but once it gets to normal running temperature the pressure drops quite significantly which is triggering the light . The garage have changed the pressure sensor, changed the oil and filter, checked the sump and are now thinking about a new oil pump. I don't want to pay for a new pump if it isn't definitely broken. It had a new head gasket a couple of months ago, not sure if that could be related. Any help greatly appreciated, it's driving me mad and the garage seem to be as confused....

 

I'm thinking Oil Pressure Relief Valve...Problem materialised on here a year or so back, can't remember out come.

 

Could be parts from old head gasket in oil chanel.

When oil gets hot valve start closing to maintain pressure. If some thing is stuck in there it can't close right. (or a broken spring in valve )

Had just change a MB Serie R 3.0 CDI engine after a head gasket change , parts from gasket block the oil to rear main bearing and total ruin the engine. 12000,00£ and you get a new engine