SoS - I do not think you are being very fair with Mad_Jock. Most airlines employ pretty competent cabin crew whose job it is to deal with passengers. They have more empathy than us, they have the on-board systems (well ours do) to tell them who paid for what, where people should be sat and a list of free (unallocated) seats. They'll also have a reasonable idea as to what is possible and normally try to use information, tact, reasonableness and charm as their tools to deal with difficult situations. We spend a great deal of money on staff training to develop these skills. But when these have failed and we get a "I demand to speak to the Captain" in the cabin I can almost predict that the outcome will not be as the person demanding my attention expects.
Yes, I will listen to what is being said by the passenger and I will listen to my colleagues in the cabin and hear what has been attempted to find a resolution. But generally, if the likes of Easyflyer83 can't sort a problem out, it's most unlikely I'll be able to do anything. Also, we have to be careful about appearing to undermine the authority our colleagues - if we are to have an uneventful flight. Lastly, I have normally found that a disgruntled passenger (a person who pays my wages) often has a reasonable case but they are generally not prepared to accept any compromises whatsoever. If they were, I wouldn't be having a discussion with them. We also don't have adequate "tools" to fix the problem before departure. Which leaves us with only one solution if we are to depart. Fortunately, I've only had to "deal" with half a dozen or so these incidents over the past 13 years (3-4,000 flights or so).
And while we are here, Mixture has added an interesting point - the "operational reasons" possibility. This generally means a different type of aircraft may be substituted at short notice. In my case we have learned to look out for those who have paid for seat upgrades, but the back-office systems have historically been out of sync with reality. Its getting better, but it can still be a challenge.
And if there are any cabin crew here, would they please tell me if a parent HAS to sit with my sons (Ages 14 & 16 - TCX flying NCL/ACE). We are happy not to sit with them and they would probably prefer to be by themselves. But one parent will be with my daughter (age 9) which means we only have to have two pairs of seats allocated plus a single. But having said that, I'm too tight to pay to select my own seat so I'll delegate the seating issue to TCX. I'm just wondering what they should/will do.