I used to have a mk1 cabrio and that suffered with saggy doors. A local bodyshop told me a trick that they use;
Open the door. Put a jack under the end of the door (use a block of wood between the jack and door so you don't damage it). Jack the jack until you start to see the car lift. Don't jack too much, take it slowly and release the pressure in the jack and check if the door now fits the whole properly. You may have to loosen the 'pin' which the door catches on (located in the back of the door shut) so that the door closes correctly into the shut. Repeat the jacking process until the door fits.
It worked for me, good luck!