How does a cruise line ensure cabins dont overbook?

Especially when they use a number of agencies including high street travel agents, cruise agents and their own promotional efforts?. We are booked on the 26 night no fly cruise from Southampton which could be oversold in some portions as it can be broken down into the 3 or even 4 sailings over the 26 night period. If one portion of the cruise was to be oversold the package could be null and void for an unfortunate customer.

I am sure they have very strict controls ensuring that overbooking doesn't occur, however, mistakes happen and that is where they would be offering compensation to people to get them to back out of the cruise and take another.

These days many cruise agents are 'live' with the relevant cruise companies. That is they can get cabin availability up when you make a booking enquiry.;)