BBM: For me, that I think is the biggest clincher. Who else knew about the pit. There's no evidence presented so far that the sons knew anything about it. The other people who knew about it could neither access it without IS's knowledge nor had motive to kill Helen and dispose of her there. The pit and, yes, the zopiclone, seal the deal for me.
I have to say I would be surprised if the sons didn't know about the cesspit. They are grown men, and I would expect them to have been informed on practical matters concerning the general operation of the services at their home - for example, knowing where stopcocks are located in case the water had to be turned off at any time. All that sort of thing.