wright,
you will need a to get the box up to a temperature of between 35-42c drain the oil and remove the sump,the filter is just a push fit but is tight make sure the o-rings come out with the filter,refit the new filter and clean out the sump and the magnets then refit using a new gasket,refill the sump until oil drips from the filler hole replace the filler plug,now start the engine and go through the gears pausing in each gear for 2-3 seconds and back to park measure the temp of the box for the stated temp range and remove filler plug WITH THE ENGINE STILL RUNNING now add more oil untill it drips from filler hole again go through the gears again and recheck the oil level if you still get a drip from filler hole you are done if not top it up until you do.make sure you are in the temp range otherwise the oil level will be wrong.
use a filtrans filter not a cheap **** copy from e-bay and for the oil you will need texaco texamatic 7045e,you will also need a pump to get the oil into the box.by doing this you will only be changing about 5=6 liters of oil out of approx 12 so maybe best to change the oil only first run for a few days the do oil and filter change run for a few more days the just oil again just to make sure you get most of the old oil out.fleet factor are doing a 20 liter drum of oil for £59+vat atm.
hope this helps and have fun its a messy job.