In any building with flats (conversion or otherwise), the weakest point is the communal entrance since it's often 'protected' with a single lock which oftentimes, is as basic as a Yale that people can get through with a piece of plastic.

Once someone is in the 'block', they aren't suspicious from the street any longer and can take their time to break into other doors. Particularly if they're smart enough to ring all the bells and see if the residents of other flats are out.

That's how they got into my place when I was burgled earlier this year. I had the 5-level mortice, the Yale and a London bar but they still managed to kick through the door since they knew no-one was in the block to hear any noise.

Other advice I got from various locksmiths and Southwark police was as far as mortice locks go, make sure that the metal 'bolt hole' is sunk into the doorframe rather it just being a faceplate. Mine wasn't.

Also it's a good time to check that any outdoor lights are working or if you don't have any to get ones that activate on approach.