It seems to be that some "key" information is being withheld. The parents said: "We know what she does and doesn't do. We were 100 per cent convinced within 30 minutes of arriving at the flat that she had been abducted." .... "We knew what the flat was like. We know what it's normally like." Also, the boyfriend was very quick to call the police. Apparently he called just after midnight. You would have to be pretty certain that "something" was VERY wrong to call that late.
There is always the chance the boyfriend is involved but I tend to doubt it. Men do kill their intimate partners but usually in "the heat of passion". Where it is planned out with staging and a "solid" alibi, there is a lot of money, child custody or other issues involved. When a guy wants free from his girlfriend, they are plenty of ways to get it done without risking life in prison. Staging usually points in a clear direction, i.e. it is an attempt to "tell a (false) story" of the crime. The "Burglary gone bad" is traditional when men kill their wives but there can be others. Staging that paints a confusing or ambiguous pictures defeats the whole purpose.
The purchase of the cider is an interesting development. From my time in the UK, I got the impression that cider was basically a "woman's" drink and it would be more appropriate for a woman alone to drink with her "take out" pizza than appropriate libations for a tryst. (Someone tell me if I am wrong).
If this was a stranger doing a "door push", it would have been someone who had "done his homework" and not just someone who followed her home. He would have had to be sure she was alone (I would guess very few young women could to afford to live alone in such an expensive neighborhood). Could he have been someone that they knew or a neighbor who saw the BF leave with a suitcase?
Apparently they had a private entrance to their flat. What is not clear is whether it opened to a discrete spot where a car could be parked and someone could be taken to that car against their will without being noticed. At 9:00 PM, I can't see someone taking someone at Knifepoint (or carrying a dead body) to a car parked on the street.
Usually, when a woman is abducted and the body is "dumped", the Perp had taken the victim, by car, to a site where the Perp feels "comfortable" to perform whatever sexual assault was planned. This site would have a means to move the victim between the site and a car without being seen.
My WAG (wild



guess) is that this is someone who either knew her slightly or "noticed" her and took some time to know her comings and goings. He did not decide to "make his move" until he knew that the BF would be away. He was probably waiting in a car for her and gained entrance either by a "door push" or some pretence. He took her to his (unattached) home somewhere in the suburbs to the west. He probably lived alone. He apparently liked pizza.