Hi,
I had that problem in two different properties.
The first one, the landlord wanted me to become one of his girlfriends and the maintenance guy was a drug addict. They both used their sets of keys to enter the flat, whether I was in or not. The first time the landlord did this, I was actually sitting on the loo when I heard footsteps outside the door of the bathroom (which had no lock!). I ended up never coming home before 10pm on weekdays, spending as much time as possible outside the flat at the weekend, and keeping virtually most of my possessions (including bank statements, laptop, CDs, suits, expensive shoes) under lock and key: I invested into a number of suitcases and padlocks! I moved out after 5 months.
The second time, workers were still 'doing' the flat when I moved in, so they would come and go, supposedly to fix the window, paint the bathroom, fit a cooker and a washing machine etc. However, if I had the day off and came back from an errand, I would find them 'resting' on my bed! The building was eventually repossessed by the bank, because the landlords were not paying their mortgage, and I don't know what became of that second set of keys! Fingers crossed, I'll be moving out soon though...