It's a good shout - and it again opens the question as to whether she knew whoever it was who entered the flat. The times of the boyfriend coming home are key. If it was the same time each day then I get they knew what time they had, but if it differed or had he come home early then they must have been prepared to deal with him too.
It would have been extremely difficult for the killer or killers to have known they wouldn't be disturbed. Perhaps they just got lucky. I say 'they' because AFAIK the police found two DNA profiles (one of which matched the DNA profile from Lynne Weedon).