Sorry to say that if it is locking up when you select lockdown 1st then you have an internal box issue again, either valvebody, geartrain failure or a seized on clutch. As far as I am aware Peter at Poole (I take it it was he) is reputable so should sort you out under his warranty.
The vehicle may possibly have a cooler flow problem as it is the return oil from the cooler that becomes the pressure lubrication for the trans, sometimes poor cooler flow can result in repeat transmission failure after a long journey (ask me how I know this

) Failure of or reduced cooler flow can cause bearings or gearsets to fail. Imagine what a crankshaft would do if it lost it's oil pressure - well the same applies to a transmission. It can be caused by resricted cooler lines, oil cooler or an issue within the transmission.
Sorry to hear you are having problems again, not what my industry needs but sadly a fact of (transmission rebuilding) life. It is how a problem is handled under warranty that differentiates the quality rebuilders from the dross.