How do you regulate influence?

http://www.guardian.co.uk/media/2011/jul/24/phone-hacking-vince-cable-murdoch-influence
You can absolutely see what Vince Cable is on about.
If one person, media or lobbying group becomes so influential with Government that it seems to be heavily influencing policy, as has been alleged with Newscorp over UK membership of the Eurozone, then you can see why there would be concerns over the effect on democracy — such groups becoming more important than the electorate in deciding what is done.
But how do you limit influence?
It would be easy to say no one group can own more than, say, 25% of the media in one country defined by the total number of people exposed to its media channels — readers of papers, TV viewers, digital media subscribers and web users etc.
But while it’s easy to enforce that prior to an acquisition through the MMC, what happens when a newly-enlarged group grows its total audience organically? Would it be right to force it to divest itself of some of its assets because it had been too successful? Should regulators intervening in the marketplace in that way?
Even before that, how can you really measure influence? The number of people a group’s media outlets reach would be a simple one, but surely some people can have huge influence without readers or viewers.
Like companies which can offer huge inward investment and want Government to regulate in a way which works to their benefit. To foreign governments who we want to stay on good terms with for non-commercial reasons like security c.f. the Saudi government over the decision not to pursue the BAE bribes investigation.
How do you contain their influence? There are no easy answers, but expect to see some attempts to be seen to contain media influence in future.