I'm new to locksmith matter and this is my first post on the site so, please, understand it if I'm asking obvious stuff or making mistakes using the thread.
I want to replace the cylinder of my front door.
The lock is a standard BS3621 Banham.
I am not 100% sure, but I think I could replace the cylinder with a Yale euro lock 45/60. May'be you want to confirm this is correct.
Now, my problem is:
the cylinder doesn't slide out after having unscrewed the one fixing screw and turned the key in either senses. When I checked the cylinder again, I noticed the kind of "cap" around it, on one side (see picture).

What am missing? Am doing the correct procedure?
many thanks in advance to those who will help for your time!