I watched this last night after recording it.
I thought it was a superb, but uncomfortable, drama.
The screenplay built up Fiona's situation very well....a bit of a wimp, totally dependent on Simon, and they had loved each other deeply once - plus her entire world (apart from the choir) was her children. So much to lose if she didn't play along with him.
The preview I saw in the Sunday Times warned that viewers would be frustrated with her reaction and that was so true.
Someone with more self-esteem and independence would have been out of the door with the kids after he admitted his guilt to her...especially the bit about smashing up her laptop...but then that would have ended the drama early on.
I reckon it was based on this investigation - although clearly Simon was one of those whose allegations of ID fraud was false.
I worked with someone who turned out to be a paedophile, and despite the compelling evidence which sent him to prison, he would never admit to himself he'd done anything wrong - that's why Simon was so adamant he was in the right. He slowly revealed himself to be the controlling misogynist that he was (just like the bloke I knew).