1. Planning permission is required if the planning officer deems that a change in the principal use of the land, upon which the caravan stands, has occurred.
2. Where those people living in the caravan use it as an entirely separate residence (and don't, for example, use the bathroom in the house), it's almost certain that it will be deemed that such a change in the principal use of the land has taken place.
3. If those living in the caravan simply use it as an extension of the house (rather than as separate from it), planning permission might not be required but this would depend upon other factors to be considered by the local planning officer.
There's a great deal more information contained within the parliamentary reply but you might stil find that it doesn't provide a definitive answer to your question. (There seem to be a lot of 'grey areas'):
http://www.publications.parliament.uk/pa/cm200 506/cmhansrd/cm051122/debtext/51122-41.htm
I recommend having a word the local planning staff and leaving them to read up on the finer points of the law!
Chris