I don't understand the question really
do you live there in London or not? if yes then you must pay CT in full register etc...and if not then there is no problem. if you live in manchester as you say you do then living with friends is never an issue. No law says that you must be living at an address. You can even claim benefit with no fix address and you can even obtain a driving licence with no fix address and be go to court and say no fix address etc...as long as you are homeless there is no issue. if you are worried about possible fraud then explain as to why you have no address .
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for the purpose of residence : Residence is not defined in law, but it has been held by the courts to entail a ‘considerable degree of permanence’. Based on this criteria, it is possible for a person to be registered to vote in two different electoral areas. A person with two homes who spends about the same amount of time in each can be lawfully registered at both addresses.
A single occupant discount is dependent on a person residencey - the two are interwined. See section 6 onwards of the Local Governemnt Finace Act 1992.
A 25% Single Person discount can only be awarded where the property is the 'sole or main residence' of one person or there are more one occupier and the other occupiers are all disregarded for Council Tax purposes.
The main gist of whether a person is resident in a property in the Council Tax legislation is 'Sole or Main Residence' which , although not definitively defined in legislation, has been clarified in case law.
http://forums.moneysavingexpert.com/sho ... ?t=1708565
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It seems clear that if you are away from home but still consider your home as in London then you must register there and pay full Ctax. if your main home is in Manchester than all your mails must be registered there. (even if you have no contract to prove you live there)including driving licences tax etc.... that is what people look for. Let's say that the Police wants to find you they would look at your registered address and approach you. unless you own 2 properties you can then decided which is which. basically your bum can only sit on one cosey place and not two unless you are married to 2 women and each have her own place