I remember watching on tv in the 70’s or 80’s a thriller where the husband was planning to murder his wife and dispose of her body by putting her in a sleeping bag, wrapped in chains and dropping it from a boat in the middle of a lake.  The wife is lying, supposedly in a drugged sleep in a bedroom upstairs in the their farmhouse.  The husband zips her into the sleeping bag and then leaves her lying on the bed, goes down stairs out of the front of the house to get the car ready.  The camera pulls back from the house and shows a young woman running down the field behind the house.  Inside the bedroom the young woman goes over to the bed, but the wife steps out of a wardrobe and knocks her unconscious.  The husband returns to the bedroom and collects the sleeping bag, believing its contents to be his wife, carries her downstairs and puts her in the boot of his estate car along with the chains.  He then drives off (being stopped on the way by a policeman who although he looks in the boot of the car does not think anything of the sleeping bag) to a lake and puts her in a boat with the chains.  He then goes out to the middle of the lake where he wraps the chains around and drops her over the side of the boat.  As she passes over the side of the boat you hear a sound as if she is starting to wake.  The husband turns only to see another boat approaching with the police in … and his wife wrapped in a blanket.  You know who is in the sleeping bag at the bottom of the lake … but he has no idea … yet!

I can’t remember whether this is a film or a tv series and I can’t remember any actors names.  If you can help me this would be great!