When i removed the engine and gearbox i noticed where oil had been leaking a little around the filler plug. I know i had tightened it well when i topped it up about a year before. I read on the forums that other owners have had a similar problem so not uncommon.
Just to give it a little more “sealability” i wrapped some ptfe tape around the thread before refitting.
Only thing to be aware is that it turns a lot easier and not to over tighten it or even poss cracking the casing.