Had this go on my TD5 defender, 2001 model at 106,000 miles (original factory fitted engine), was the oil pump bolt, threads had gone on the bolt and it just came loose! oil light came on and apart from a slightly noisier top end, you wouldn't know, thankfully i wasn't going fast and i stopped immediately so there was no other damage from lack of oil.
Wasn't a cheap fix, if I had known I would have checked it as soon as i bought it!
Would have a looksy if I were you!