Maybe not an attic collapse but I have certainly seen cases of junk shoved into the attic (and not on the ceiling plasterboard itself) causing the ceilings to crack. All I am saying is proceed with caution. If the items being stored are (almost?) too heavy to lift up in the first place then be careful of where and how and for how long you store them in the attic.
agreed. Floor attic to prevent such things or better still hire a skip for a day and throw all the crap away or boot sale. As someone said once it's up there it will probably never be used again.