You can get 5.5l out by removing the rad cooler pipe and starting the engine for 30 seconds.

You would usually do that before removing the sump.
Refill through the hose using a pressurised container full of oil.

Where you are now you are best off putting it back in through the filler hole.
Once it starts to leak out start the engine and continue filling and put plug in. Stop engine.

See guide here on how to change the fluid held in the TC too :

http://www.disco3.co.uk/forum/topic55010.html


And info here on changing the sump as well, several posts down;

http://www.disco3.co.uk/forum/topic144344.html

Ok here's the update. I managed to drop around 3.5 Ltrs of transmission fluid from the sump plug. Once I'd dropped the gearbox pan down I managed a total of just 5Ltrs plus minimum spillage.
One mistake I made was to remove the sleeve without actually unscrewing the connection plug on the outside of the box, a litthe grunting and head scratching , solution, really install and unscrew then remove the sleeve.sorted.
Rebuild involved locating the lug on the inside of the sleeve and a small amount of leverage to get the sleeve homed in.
Rebuilt and new oil time, the best solution I could find was a hand oil extractor pump, works ace just required a assistant to make the job easy.
A total of 5.5Ltrs of zf fluid went in following the original instructions at the start of this thread curtasy of rrphil (Phil) thanks.
Also, on the plus side the date on the old pan indicates a new pan was installed between 2014 and now as it had 2014 stamped on it.