As per the OP, BS5839 states that as fire alarms are a safety system, cables must be segregated from all others and therefore have there own containment or segregation within tray or trunk.
In reality interference will be minimal if kept with ELV circuits but that's what in stated in the standard.
Not seen Scotmod around for a while, so bumping this old thread may not answer your question. However, the way I read it is that by conforming to BS7671 you cannot get interference, I don't agree.