Ask dlr to check over completely, this is a very worrying fault matey, as far as i am aware, these things are fitted in at build, and CANNOT be replaced. If as Wapping suggests, it is the sight glass & not from somewhere else of course?
As i said before, they only EVER leak/come out if the engine has been seriously overheated.
I'm not trying to scare-monger, just look after your 'hard earned'.