Had mine done not too long ago. No dramas, all very pleasant (
Firearms and safe inspection a breeze)
On the subject 'just turning up'. I know of it happening both ways.
My guy called a week ahead, made an appointment on the weekend with me. Easy done.
A mate of mine changed jobs and lost his company phone, changed numbers etc. and the fuzz weren't able to get hold of him. Due to that they made a visit unannounced, as it happened the timing was good and they did it anyway. Extenuating circumstances in that case but it happens.
In either case, police do not have the right to enter your home for the purposes of a standard firearms and storage inspection. You must invite them in.
If they turned up on your door unannounced you would be completely within your rights to refuse them entry at the time and agree upon a later time/date for the inspection.