Today I was talking to the transmission specialist, who is rebuilding my L322 auto, about the Disco's reluctance to lock the torque converter. He suggested that the transmission would be the last place to be looking for causes.
He said the first places to look would be MAF, MAP and TPS. He has known problems caused by petrol engines running on E10 instead of 91. Any slight hicoughup, even imperceptible, can send the lock up into a tizzy.
Before spending big dollars on the transmission or converter, make sure the engine is in peak condition.