Aerial power/freeview dignal poor issue

[Capa]

My mum lives in a block of flats. She has one freeview box in the livingroom (sitting room) with a perfect picture. She has another freeview box in the bedroom. The freeview box in the bedroom doesn't tune properly. The few channels that do tune stop start and freeze alot with "BAD SIGNAL" appearing on screen. I'd really like to get this fixed for my mum.

Things I've tried:

• Moving the working freeview box from the livingroom to the bedroom. The same problem occurs with that freeview box.
• I opened up the TV socket on the wall to make sure the connection was nice and tight. (it was but i took it apart and rewired it anyway), this made no difference.
• I tried a completely different wall socket in the bedroom but again, this made no difference.

When I look at the roof I can see one huge (I presume digital) aerial shared between 8 flats. Presuming each flat have a couple of TV's in them, I'm guessing there will be a signal booster or aerial amplifier as standard on these new flats.

As my mum is in the top floor of the flat, I managed to get into the loft and did see the aerial cable running into the bedroom and the livingroom but there is no sign of any aerial boosters or amplifiers. Infact I could not see where the aerial comes into the loft from the roof.

Does anyone know why this bedroom signal is so poor and what can I do about it?