richard when you built your dwarf wall with the cavity your outer skin should had been built seperate on the door opening from your inner skin,for instance a brick then some dpc layed vertical against back edge then your brick or block work inside,you lay ties across cavity about 150 away from edge of opening to tie the two walls together , hard too explain unless with a photo like you said, i wouldnt bother putting in damp now long as you have the damp above ground should be fine ,