X5 - Possible Issue???

[The Loon]

I have a '56' X5 3.0d Sport (Old Shape), and this morning I started it up left it running for 2 - 3 minutes, when I went back to drive away a message on the dash read 'stop engine - low oil pressure', so I stopped the engine and started it again and the message had cleared. Thought I better check the oil on dipstick and it was a touch low so just put a splash of oil in it. Started again and no message, drove all the way to work this morning and stopped engine and now 'check oil level' is on the dash.

Have you guys got any suggestions??

Cheers