I saw those large bolts previously but they are not "highlighted" in the manual illustration so assumed they were not necessary to be removed. I'll remove them next.
Update
Went down to the garage later on this morning and had a closer look at the 336. The two bolts behind the differential referenced by John do indeed go all the way through the case. I missed that during my initial inspection, feel like an idiot. Anyway the two bolts are now removed and the case is loose but not completely apart. I'll attack that part later on this week. Thanks for the clue.