That would likely depend on the engineer who is assigned to you, and maybe even what they think of your job from the outside and their workload for the rest of the day.
I don't think they are obliged to call you at any point, though many that I've encountered have. If you get a engineer who isn't as conscientious as most and your job looks like it could be more difficult or time consuming than others (particularly as you say it's a top floor flat), and/or they have a load of other jobs still to complete, then they may do the bare minimum of ringing your doorbell and then recording that they couldn't get access. That type of practice seems to happen more where subcontractors are involved.
Hopefully you'll get a good one and they'll call (maybe even to let you know they are on their way), or you manage to spot them yourself when they arrive and it will all go smoothly.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
