Disco 2 Torque Converter lockup too early, and while going up steep hills. Intermittent.

I have a 2001 td5, which I've had a few months. Previous owner had health problems and didn't drive it much over the last 5 years. On drive home from collecting it noticed a bit of a grating sound which I recon was coming from tc. Replaced oil and filter as soon as I got it home using correct procedure. Only 2 and a half litres came out, and it was a bit dark, so replaced again after 100 miles. Grating sound almost disappeared after oil change, and it's been driving pretty normally apart from a bit of spazzing out when I was running the AC (have since extended the ac drain pipe away from the xyz), no shudder, and the TC has been locking up normally.

However. Did a 5 hour drive through mid Wales today. Going up steep gradients with the car working hard, the tc kept locking up going at about 35 mph, until I floored it at which point it kicked down, and out of tc lockup. It would also seem to intermittently stick in tc lockup going downhill at much lower speeds than it's supposed to.


My guesses are
-xyz switch (planning to take this off and service it at some point soon but haven't got around to it yet
-I'm guessing there is a solenoid valve or something that controls the lockup that might be sticking?
-My gearbox is rhymes with ducked.

Any help would be very much appreciated, and I promise to check back with updates / resolutions / financial ruin...

Other things to mention.
good: it's done 140,000miles. It's not done any towing. It's been pretty well looked after (insane amount of receipts with it).

Bad: I don't get selected gear notifications on the dash. I don't even get the warning lights flash on ignition, so I recon the BCU currently thinks it's a manual. Planning to hire a nanocom and confirm at some point. The positive terminal was loose on the battery, and I read somewhere that this can freak the BCU into resetting itself or something.

It had a recon turner engineering block and amc head in 10k miles ago. But I wouldn't rate the outfit that did it though. Things like half the intake manifold bolts missing, engine mount bolts missing, crossmember bolts missing. Some incorrect bolts used on rocker cover etc make me think they were a bunch of jokers. I dont know if there is anything that they might have done wrong while the engine was out that could cause the symptoms, but thought I'd mention.