They may not be able to use the same theme music, or even remix it. Often TV broadcasters commision the composers for theme tunes for TV programs.
If this is the case for the BB theme, it would mean the music belongs to Channel 4.
Looking at releases of it on CD etc,
http://www.amazon.co.uk/Big-Brother-.../dp/B00004Y33K
http://www.amazon.co.uk/Big-Brother-...=pd_sim_m_h__1
It is released on the Channel 4 Music label, so chances are it was a commissioned work for Channel 4. Oakenfield and Grey would either have received a flat out fee or may get annual royalties.
Even if it was a work commissioned by Endemol itself Ch5 will still have to ask Ch4 for permission to use it. As Ch4 Music hold its copyright.