If you'll indulge me, just a few random thoughts I'd like to throw out (apologies if you feel these have been dealt with):
Do we know if any of Helen's family or friends corroborated IS' story to police that Helen had had three swanky wedding venues 'fall through', one of which was Brocket Hall? This is not hugely important but as someone else has said, she and IS did not appear to socialise together much/have many mutual friends and he was clearly at pains to keep their wedding plans private - to the extent of not even telling his sons they were engaged. To me, this does not tally with a big expensive wedding at Brocket Hall? It could be that Helen was going against his wishes and wanted a big, pricey do. As he had no wish to/intention of marrying her (and certainly wouldn't want her paying out sizeable deposits for various wedding costs), her bridal plans could well have hastened his murder plans.
IS had only 28 sleeping pills from his GP's prescription. As Helen's drowsiness was becoming extreme by April, he must have been getting those pills from another, additional source, probably online - this is presumably one of the reasons he wiped the computer and 'lost' his phone.
Why on earth did he hatch such a muddled and ultimately doomed-to-deduction murder? Is he stupid? Did some element of his plan go wrong, so he was forced to botch it?
It seems to me, pretending she was missing was not in itself a mistake - but claiming that when her body lay in the cesspit of their house was utterly ludicrous! He has surely handed the Crown a murder conviction on a plate.So did he originally plan her demise in some other way, ie hoping to make it look like accident or suicide, with the help of the sleeping pills he was secretly feeding her? I just cannot understand why he dumped her where he did - if her body had been found at the Broadstairs house, or he had even staged a car accident - or any number of different scenarios, he may have had a chance of escaping detection. But by killing and dumping her and Boris in a cesspit within the house, it is a clear case of foul play and the most likely suspect was always going to be him!
I won't ramble on but I'll just add - I wonder if he regrets his actions now he is facing life in prison? Does he ever think with regret of that warm hearted, witty, sweet and funny lady who loved him enough to marry him and share everything she had with him? Does he look at pictures of the garden where he dragged her dead body, see the swimming pool and swanky conservatory and imagine the luxurious retirement they could have enjoyed together if he hadn't carried out his plan for her destruction? Does he wish his sons and elderly parents still saw him as a wonderful dad and only child, rather than a soon to be convicted murderer of a woman and her dog for financial gain? He will never face the fact he is evil but I can't help but wonder, in his current, desperate predicament - DOES HE EVER WISH HE HADN'T BEEN SO DAMNED GREEDY?