LT77 gearbox leak

I have found the source of the river that flows from my gearbox. If you look up past the High/low selector pushrod towards to front of the main gearbox lookings backwards (Still there or confused?). There is what appears to be a large (25MMish dia) blanking plug with what maybe an allen key hole in it. What is this plug, Is it mystical in its function or can I just remove it and bung some thread sealant round it and hope for the best, once I've worked out how to get an allen key to it. Any ideas