Help! Could my house insurance be invalidated?

[annavc]

Not sure this is the right section but maybe someone can help. I live in a rented shared house and the 3 of us that live in the main house have a standard household contents insurance policy. The attic of the house is a seperate bedsit except for access which is through the main front door and up our stairs. We have just discovered that the woman who lives up there has given her mother a key to her flat but also to our front door and she uses this regularly to let herself in whether the woman upstairs is in or not. We can't lock our bedroom doors and our posessions are all over the main house anyway. We are now wondering whether our insurance would be invalidated by someone who is a total stranger to us having free access to the house and therefore all our belongings. We're really angry about this but want to get our facts straight before confronting her about it and then possibly involving the landlord.

I'd be really grateful for any help anyone can give on this. Thank you.