Too many things don't add up to me, though in the actual book they may be explained in more detail.
I'm not convinced that she knew his house; sure, the taxi dropped him off there, but she was looking the other way, or at the money he left, so I'm not convinced she knew the exact house. And houses of that style are usually divided into flats- unlikely he'd own the entire house.
So how does Mr X know which it is, and how does he gain entry through a buzzer intercom (assume) unannounced? And how does George 'just happen' to be at home?
All dubious.
He's not a spook. If he were, he'd not even hint at it. He'd have an alias identity. You don't 'hint' you are a spook if you really are!
I'm not convinced the man he brought along to 'help' her decide whether to report to the police was genuine either.
I also don't know why he made her trek to SE11 to the flat. What purpose did that serve? They had sex, but they could have talked and had sex in a hotel. There are cameras everywhere, so you can't argue he'd be spotted more easily in a hotel lobby than travelling to SE11 from the west end.
I think she is coming over as very naive for someone of her intelligence. I'd hope the book fills in these gaps or it's rubbish.