Thanks. That picture makes it really clear. Looks like it'll be a bit of a faff but do-able.
Just spoke with my dad and he said his Saab garage implied the gear box oil is one of those jobs that can pretty much only be done one way so I guess it's just a matter of getting on with it. Still, if anyone has any recommendations then please share them. I will definitely be following the tip of making sure the refill plug is removed first before draining the oil!