or you could try the freeview box from your lounge in the bedroom and vice versa so you know it's the cable not the freeview box , some boxes are better than others picking up signals,
You can also connect the weakest cable(lounge , then disconnect the other cables from the splitter to find out a) which is feeder from the aerial and b) if anything is interfering with the signal
just reread your post.......you only have one freeview box ? yes?
It might be easiest to run another cable from the cupbord to downstairs somehow(under the carpet if it's only temporary