Hi
I replaced my TD5 (2001) Disco hub assembly a year ago. The most likely cause is that "your mechanic" hasn't reseated the sender in the back of the hub. I had the dreaded 3 Amigos and had to remove the stupid little rubber washer that seats around the sender. Use a narrow bead of good quality silicone to seal the sender. The amigos (as in my case) should reset themselves.
I am near Mintlaw, Peterhead Aberdeenshire, If you want any more advice.
However, look on threads on Landy Zone for some great advice.
Hope this helps.
Andy.