Leaking waste pipe

I am in the process of converting a bedroom into a bathroom. I have run the waste from the toilet to the soil stack, I have connected to the stack but I have found that the 4"/110mm joint is a little bit too big for the soil stack and so the joint leaks. I haven't used this in anger, just tested with water! What is the best way of sealing this joint. Is it worthwhile going for plumbers mait / silicone or perhaps a mortar type fill. Or start again? I also have found another hole in the stack which needs plugging, would the same technique work for this. Any help would gladley received. Thanks Bean