as per the manual I undid the 21mm nut and found that it took a lot of jiggling and force to remove the plug and oil screen. It got stuck at about 3/4 of the way out and wouldn't go back in either? I ended up applying a lot more pulling power and it popped out intact, but I was lucky I'm sure. Why is it such a tight fit? Still unsure how I'll get it back in? Is it worth carefully filing down the plastic end piece edges for easier fitment? Also, only about 1 1/2 litres of oil has drained, even though it was correct level on the sightglass, do I have to remove the filter to get the rest out? Or is there another drain plug not mentioned in the manual. Thanks for your help.

There is a brass plug that will take a 10mm hex key on the underside of the sump, almost directly behind the filter screen and close to the exhaust pipe. They can round out easily so make sure you have a good fitting key. I would have thought that most of the oil would have come out through the filter screen plug though. I usually drain between 2½ and 3 litres if the oil is up to the mark and a bit more when the filter is removed, with a refill taking just over 3 litres.
Not sure about the plastic screen. They can get a bit distorted if they have been refitted without the end piece properly into it's mating aperture inside the sump. Maybe that's what happened to yours. If in doubt buy a new one. They should still be available as they were fitted to all Ducatis until they changed to the 3-phase alternator, around 1998, at which time the crankcases were redesigned.

Interesting.
I have never had a problem with this. Maybe someone had it to tight to begin with. Check the shop manual available on this site for the proper torque.