I'm not sure I follow the logic of any of this ...
box downstairs - works ok
box upstairs - no workee when connected to new telly...
qed ... (no that's not on any more!) either telly faulty, box is faulty or Aeriel is faulty.
(is box>telly connected by scart?)
aerial connection to box in>>>box out>telly
it can't be signal reception strength - as it was OK downstairs??? ... if I understand there is still only one telly connected to the aerial ... so nothing has changed except the location of the receiver.
is digital TV reception OK downstairs?
if yes .... it's the lead to the bedroom
does the bravia work downstairs .... if so ....
if reception is poor ... the connections will need to be spot on ... stick hubby in the loft ... don't let him down 'till it's all checked out.
consider an amplifier rather than a splitter (or is that what you mean by splitter??)
(this sort of thing
http://www.beststuff.co.uk/aerial_amplifiers.h tm