As heading really. Looking at buying a car and an old for sale add on here says the oil separator is leaking - I don't know if the work has been done but what is it and is it A) bad and B) expensive to fix?

 

Was going to describe it but this should be a little more helpful... If you do replace it you can buy a metal plate instead of a plastic one which is worth getting

 

neither are an issue if they are sealed and fitted correctly..................

main problme is its behind the flywheel so its an engine or gearbox out job to access the clutch/flywheel assembly and then re-seal this plate

 

It is an issue because you are leaking oil and it may contaminate your clutch which will ruin it. It is not something that should be ignored however it is unlikely to cause any real engine damage it will just make a mess, loose a tiny amount of oil and maybe cause your clutch to be ruined.

As said the main pain is fixing it. Parts are cheap but the fix involves engine or box removal. Engine removal is by far the easier of those two. It will obviously incur a fair labor cost.

Lastly.... how do they know it is leaking? Unless they have taken the box or engine out you can't see if its the plate leaking or the rear crank shaft seal. Both of which are real possibilities. If they had gone to the trouble of taking out the box or engine then they would have fixed the issue. It may be a guess as to which is actually leaking.

If its the rear crank shaft seal it's still a cheap part but a fiddly job without the proper tool and requires the same amount of labor.