I feel for you and completely understand where you are coming from... my sisters children are very similar...spending time with them all is a chore and she is my sister. The only difference is that my sister tries and will tell them to stop interrupting, but shes too soft in her approach.
I have to agree with anon and CW, i dont think you can really say anything... might be worth a try to tell the child yourself (may also be a complete disator!!) Next time he interupts you, tell him that youre trying to have a conversation with his mummy right now, do it with a sickly smile on your face but open your eyes a bit wider when you look at him, as if to say 'shut the he*l up' with your eyes!!