THE row over rescheduling News at Ten has split the ITV companies along roughly the same lines as the debate over mergers between the nine large regional franchise holders. The largest companies - led by Carlton, Central and London Weekend - are in favour of both changes, while the middle-ranking companies of southern Britain - Meridian, Anglia and HTV - have their doubts, and accuse Carlton of trying to dominate the system. Which prompts a Machiavellian thought. The word is that Peter Brooke, the Secretary of State for National Heritage, plans to lift the ban on mergers between large companies, subject to safeguards against the two London franchise- holders getting together and possibly against Scottish TV falling into non-Scottish hands. He has not decided when to announce it. What if, responding to political pressure - including yesterday's intervention by John Major - he were to make it known that he will delay his decision on mergers until the fate of News at Ten is settled to his liking?