Firstly I apologise if this query has already been covered. Up to a month ago, my 03 TD5 started first turn. Now it has occasionally decided not to crank. Dashboard lights are correct when ignition is turned on. Glow plug light goes out after a few seconds, but when I turn the key to crank...

Firstly this is my first posting, so hello one and all. Now my 03 Discovery facelift TD5 has "developed" a quirky habit of flashing the oil warning light when descending a hill when I need to slow down. ( Camera etc). Now this only happens on the odd occasion, but it sure as hell doesn't do much...