Dropped the engine under-tray today to change the engine oil and filter and noticed a slight oil leak coming from the small end cap just below the gearbox filler.

Does anyone know if this cap can be replaced externally or will it involve being fitted from inside the gearbox which means a strip down.

2016 2.0Ltr Tdi 6 speed manual NFZ box

Thanks in advance.

Not entirely sure but if it were me I would smear a compound such as Aryldite around the circumference of the plug and observe it over time for leaks

I agree. I have purchased some RTV silicone and will give that a go. 

I called a transmission specialist who suggested the same as you. He did say it shouldn't be a problem as it's not under pressure and there is no oil seal behind it. He did say he doesn't even know why it is there!

If the sealant doesn't work I will try Aryldite.

Curious about your username.....what bike do you have?