Ross Kirkland
I presume you mean 5mm gap, if so, it's likely a combination of worn gaskets, poor original installation and possible hinge failure. If the windows are tilt and turn, there should be adjustment in the hinges using a combination of turning a small adjuster wheel and an allen key (may have to remove a hinge cover first), if they are hinged at the top and bottom (opening out the way, then the friction hinges are possible broken. Windows can be tricky and the units are heavy if you have to remove them to fit new hinges, you would be best to get a professional to take a look, you'll find a trades person by posting in the job section. Hope this helps