I've found a PVC soil pipe which is basically resting on the clay drain collar with what appears to be a solvent weld coupler. There's no evidence of it ever having backed-up but it can sometimes create a smell. Obviously a drain connector should be in the clay collar then the PVC pipe dropped in to that, however that's going to be very difficult as the whole stack is going to be left unsupported if cut and a slip coupling would also be needed. Therefore I wondered if a foam or paste type product could be used to fill the 1" or so gap around the pipe to the edge of the clay collar and if so what to use. Basically create a seal around the collar (which was filled with lose sand) before back-filling with screed. Many thanks indeed!