I wouldn't worry about that. In my opinion, complete sentences are a joke in fiction. You're not writing a research paper here. Good prose conveys meaning in a way that flows well. Sometimes complete sentences are counter-productive.
A long sentence is not necessarily a bad sentence. Just make sure it is a well structured sentence with commas in the right places, and make sure it sounds good to the reader. You can write a long sentence if you do it properly. One of my favorite writers, T.R. Pearson, often writes sentences over 50 words long.
Good paragraphing; be sure of one concept per para.
In this piece there are some places where I think you can actually combine some of your paragraphs.
Run-on sentences with syntax/tense conflicts.
"Unwrapping the objects inside the box she found herself lost in reminiscences of her past “glory days”, the uniform of her ragtag army, Black jacket, and urban camouflage combats and the belt with her little friends of ammunition grenades and climbing apparel attached"
This sentence could be restructured, but I wouldn't call it a run on sentence. There is a little confusion about the list of things in the box for me. Putting your commas and "and"s in different places might help. I might write it something like this:
As she unwrapped the objects inside the box she found herself lost in reminiscences of her “glory days” (note: glory days implies past), the uniform of her ragtag army, black jacket, urban camouflage combats and the belt holding her little friends: ammunition, grenades and climbing apparel.
Do the black jacket, combats and belt describe the uniform or is the uniform something separate? If they describe the uniform it would be a little different.
Too much imagery.....yow! I'm going BLIND!! Simplify and let the reader see it their way.
"friendly rabble eating joking, the rustle of pages turning noises, aromas and vivid colours all together clamour for attention in a cacophony of sensory stimulation."
I disagree. I think this descriptive sentence sets the reader up nicely for the moment when the package is dropped on the table and it seems to be the only sound in the room despite everything going on around. Maybe move those two parts closer together though.
Avoid alliteration. Always.
Only Hemingway could do this, albeit badly. Hemingway!
"a rickety run down reminder of the concrete obsession of the nineteen nineties" was one of my favorite sentences of the story, though I'm not sure I would describe concrete as rickety, no matter how run down. You do a nice job describing a drab urban environment.
Watch your tenses in the last sentence of the first paragraph.
Try something like this: "No longer working, they had become a haven for addicts, their floors (note: there is more than one lift, right?) littered with the paraphernalia of substance abuse."
Keeping tense consistent is difficult for me too.
This piece requires a ton of editing.
I don't think it requires as much editing as others seem to think, but it definitely could use some. Don't be disheartened by that though. I'm sure Updike didn't get his stories right the first time, and probably not the 10th time either.
I enjoyed this. I think you've got a good start.