Help gear box oil leak

Hi everyone, could I please get some advice.

I steam clean my Landy about once a month so I can keep an eye open for leaks.

Cleaning the engine, rad and inter cooler has proven to be brilliant as iv solved loads of problems.

Recently I noticed all of the gearbox covered in oil and grime and over the past week it's begun to clunk when changing gear.

Having steam cleaned it today, iv realised that being new to landrover a means I don't know anything about the gear box, not am I a mechanic.

So how do I find out what box I have?
What oil should be in it?
How do I top it up?

Mine is a 1989 110 defender, 200tdi with a Salisbury rear axle.

So please what do I do now?

Small update,

From searching iv found out that I have is believe to be a LT77 box.
So now just need to find out the common places for these to leak from, how to check and top it up and what oil to put in it.

Thanks in advance again for any help.