I got it in my head that I need to check the gearbox oil. I know that the box is getting tired, but I thought it might be a sensible thing to do. Looking at haynes, there is not a lot of information on how to do this. Is this even a sensible thing to do?

 

I've noticed that there's a plastic plug that is very loosely covering a hole. It's on the top-front of the gearbox/clutch housing. What is this for?

https://imgur.com/a/r8ExSnc

It's for checking the clutch so you can get a camera on a stalk down there to diagnose wear and faults.