I asked a similar question a few years ago and discovered that if it's less than a couple of months then I think you'll find that most will accept you. If it's longer than that then most will move you on because you will effectively become a resident and they don't like that. Some marinas will accept you as a long term resident provided that you are discrete and in my experience this means talking directly with the marina manager.
I'm sure that someone more qualified than me can give you the legislation on this. Hope this helps
I believe that this is the gist of it. Essentially, marinas are not viewed as residential sites like say a permanent caravan park. Although come to think of it with all those modern plastic palaces (Olympic flames my boatyard calls them*)perhaps they ought to apply!
I've 'lived' in several marinas in the UK: Kemp's in Southampton was one, but so long ago (well before poll tax) that I can't vouch for it now.
Unlike Chris I have always started by simply signing the normal agreement prohibiting live-aboards, and quietly got on with it. Taken the boat out occasionally, to show willing. NEVER ask permission, as it can't be given, nor deliberately drawn staff attention to it. Keep complaints about facilities to a minimum. Don't fix up anything houseboat-looking, like a permanent TV ariel.
After a bit you invariable realise you aren't the only one, and of course the staff will rapidly know about you, but mostly they are willing to turn a blind eye if you don't rub their nose in it.
However, very occasionally the local council will have a purge, and then the marina get touchy, so you might have to go on an unplanned winter holiday. A few councils keep a constant check (IOW, I've heard?) so living aboard in those areas really isn't practicable.
People aren't too free about saying where, since marinas can't afford to establish a reputation for allowing liveaboards, and no-one wants to queer their own pitch. One thing you can do to check is to visit a marina in the evening, mid-week, out of season. If you see cabin lights here and there, the odd TV arial hoisted, one or two people on the pontoons when the pubs close - the marina tolerates liveaboards.
Incidentally, many standard yacht insurance policies prohibit living aboard.