Does the aerial feed go into YouView box first?
Have you tried moving the aerial cable away from the HDMI cable?
How old is the aerial flylead - the cable that plugs into your aerial socket on wall to YouView box?
I've found that YouView can be very temperamental to the quality and route of this cable.
You could always try a cheap signal booster from e.g. amazon, argos, maplin etc.
but I'd experiment with the moving the aerial flylead around in the first instance to see if that helps.
I ended up having to use a shielded flylead but even then if I move it a few cm I can end up losing the freeview HD channels.
Take a look at: http://www.digitaluk.co.uk/help_and_advice/engineering_works
to see if there is work going on in your area before you try a booster.
Also check if you have any duplicate channels on your YouView box - I think they are normally in the 800s range - in case it has picked up signals from more than one transmitter.
You could always try a channel rescan to see if that helps.
Curious that it worked before but not now which suggests that something has changed.
We had a similar problem and also live in an apartment with a communial aerial. We have a TV socket in the bedroom that got its signal from a "piggyback splitter" from the lounge socket. What I did was to plug the youview box directly into the lounge aerial socket removing the "piggyback splitter" and plug the cable to the bedroom socket to the aerial out socket into the youview box. This cured our problem. Hope this makes sense and is of some help in your case.