They couldnt possibly remove all the oil.  38ml is quite a lot for a regas, it certainly sounds enough!  You would probably only need to put the full amount in after a full flush or full replacemnt system.  I wouldnt worry!
As for the filter, most cars have a cabin/pollen filter but I cant see any info on one in either the 300tdi or the td5 discos so it appears not.