UK Car Repair: Oil Leak Corsa 1.0, crankcase breather, oil combustion

Question
I have an oil leak on my 1999 Corsa 1.0, had the sump gasket replaced, still leaking.   Now had the crankcase seal replaced, leak still there, but not as bad.  Any suggestions on what could be causing it, or do you think the garage may not have fitted the seal correctly!

Answer
It sounds very much like the engine is building up too much back pressure.  All engines today have a crankcase breather system that allows the oil/combustion vapour to be fed back into the intake.  The system includes some sort of filter, find out where it is and clean it or have the same mechanic do it.