Okay....Thats a lot Thanks for the advice:
"Depends very much on what type of door it is. On a rail-and-stile door it's normally not a problem to cut off 2.5 cm (1") on each end.
Also a solid core door would be fine. In some doors the core is made of corrugated cardboard and that could be a problem. The rails are often quite thin and if you take off too much the core will be exposed."
...and all the rest.
PS. I'm from the uk...and I am young (and clearly more inexperienced than I thought)
Thanks for all your suggestions guys.