L live in Kenya and drive a Tdi300 Landrover defender 1998 model. I had the same problem 4 months back, the mechanic fitted a gasket but the leak started again. Today I removed the gasket and carefully fitted a new one with a generous application of silicone sealant. True the sump is as described by Bogmonster. I was careful not to overly tighten the sump bolts. I have left the silicone to cure over 24 hours.
Will post the results in 7 days.
Question is, why do the spare parts stores stock the gaskets?
Who manufactures them and why?
Am confused.