loe oil pressure!!

Hi i have an 86 defender with a 200tdi motor in it. i was at a play n pay day at coney green when i lost oil pressure! normily 35 to 40 psi at idle when worm, now bout 10 psi at high rpm! iv checked the normal things, shells, oil pump they all seem to be sound. when i lost pressure i turned the engine off strait away. any idears??:doh:

checked oil level and conditon?Not done any wading have ya?Bit of water in the will upset things.

yes i had been waiding, quite deep but i have raisd air intake which is sealed as well as all breathers. and since iv had the sump of to cheack shells its had new oil and filter! im just about to go and check oil pressure relise valve in filter housing!