Land Rover Repair: TD5 MIsfire, air leaks, engine harness

Question
I have a Sept 2002, 62,000Mile TD5 that has recently developed a misfire and proving difficult to start - no power top speed only about 50 miles an hour and struggles up hills. Lots of oil in the ECU connector and Injector Harness so I replaced the injector harness and cleaned out the Engine harness and ECU connector but made no difference in fact the problem has become more persistent. What else could it be before I take it to Landrover for a diagnostic check ? Oil in the ECU itself ??

Answer
If oil travels up the harness and gets into the ECU it wire surely cause trouble.  Why don't you check that before going to the dealer.

I assume you have also changed the fuel system filter and made sure it's tight and free of air leaks, right?