Gotcha, thanks. I've been reading up on Filo's flushing process (on Disco3 UK) and think I might be best with this process:
- Disconnecting trans fluid line into radiator, then turning engine on to drain what I can
- Turn off engine, drain sump, then replace plastic transmission pan with metal pan and new filter kit
- Top up transmission fluid using pressurised hose using correct cooler lines near radiator (again, following Filo's method)
- Top up transmission fluid using Filler plug in transmission, run through gears, etc 
Expecting now to use more than the 7L that came with the metal sump conversion kit, so will need to purchase another 3 or so Lifeguard 6. 
Would appreciate any pointers on these steps. I've looked through both this thread, and the thread on Filo's posting, and cannot see the process detailed like this.