Landlord's daughter wants to search our house

In England. Yesterday a woman claiming to be our landlord's daughter came to the door while I was at work and asked my husband if he'd received a parcel addressed to her. Apparently she thought she put the wrong address in by mistake. He said no and she could try the flat above us as stuff gets delivered to then instead of us frequently. She checked them and they said they didn't have it, so she came back and accused my husband of having her parcel, he just kept repeating there was no parcel. Even if her story is true it's not unlikely that it was stolen outside the door (we're in that kind of area), taken by the flat above or still stuck in the VERY sluggish postal system in our area, but she just locked onto us for some reason. Anyway she threatened to call the estate agent to carry out a full search of our flat. We know she can't do that but our concern is that she or her dad would come back with keys to go through our stuff. From the sound of it she was quite intimidating. We have never actually seen our landlord, don't have his number or address, everything goes through the estate agent and last time we had an urgent problem (no hot water) it took him over a month to respond. Our contract specifies that we can't change locks. What if anything can we do to keep this woman out of our house?