heeelllooo and welcome monkeyhouse
quite normal to be seperate
the back will be perfectly safe once loaded up unless you have a bouncy floor
i you are worried about the top falling over
a couple off these upside down on the top assuming there is a flat top or an area you can screw too
http://www.screwfix.com/p/corner-braces-zinc-plated-51-5-x-51-5-x-16-35mm-pack-o...with the top in place mark the wall where the brackets are going
place the bracket 5mm above the pencil line so the leg coming out from the wall will clear the top slightly
make sure the other leg goes down so its covered by the unit
affix to the wall with 2 screws
the top will be around 1" from the wall assuming you have skirting stopping going back so just screw through the outer hole
if the floor is bouncy then a bit off string with 2" off slack would cause less problems