I trod on the threshold and most of it split away, fairly rotten as was the remaining piece. I obtained a suitable ,cross-section wise piece of second-hand hardwood. With template etc I should be able to cut to fit. I have some slates and also some reconstituted compposite slates and am thinking of placing a slice under the hardwood and over the underlying concrete step, as a DPC. Good idea or not?, and use cut down slate or the modern "slate" ? I'm thinking the modern material would take any local pressure points from the concrete and human weight on the timber , better.