i presume this is a td6, on top of the engine at the back is a breather system housing which has got a filter inside. the unit is supposed to be changed annually and only costs 30 quid, thats most probably the cause of the pressure build up. get it changed and see if that helps the smoking issue aswel, if you need to see a picture so you know what to look for go on famous four web site and go to the 322 section and look under service items.