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I did try and post a link but couldnt so sorry for the ammount of text but this is a really good explaination of the "powers" of a citizens arrest .
A citizen's arrest (officially called an "any person arrest") is permitted to be made on any person under section 24A of the Police and Criminal Evidence Act 1984 for an indictable offence, including either way offences (in this section referred to simply as "an offence"), but excluding certain specific ones listed below. It is thus permissible for any person to arrest:
Anyone who is in the act of committing an offence, or whom the arrestor has reasonable grounds for suspecting to be in the act of committing an offence, or
Where an offence has been committed, anyone who is guilty of that offence or whom the arrestor has reasonable grounds for suspecting to be guilty of it
In order for the arrest to be lawful, the following two conditions must also be satisfied:
It appears to the person making the arrest that it is not reasonably practicable for a constable to make the arrest instead
The arrestor has reasonable grounds for believing that the arrest is necessary to prevent one of the following:
The person causing physical injury to himself or others
The person suffering physical injury
The person causing loss of or damage to property
The person absconding before a constable can assume responsibility for him
Use of the second power above is rather risky, since it relies upon the person carrying out the arrest knowing that an offence has been committed, of which in itself needs to be indictable or either way offence. The Act therefore gives a constable additional powers under section 24 to arrest the following:
Anyone who is (without doubt) about to commit an offence, or whom the constable has reasonable grounds for suspecting to be about to commit an offence
Anyone whom the constable has reasonable grounds for suspecting to be guilty of an offence which is merely suspected to have taken place
N.B. 'Any person' powers can be used to arrest before an offence occurs as long as the offence in question falls within the Criminal Attempts Act 1981. This act creates the offence of an attempted offence, as long as the offence being attempted is an indictable one. For this to apply, the offence must actually be in the process of being attempted - preparatory steps are not sufficient. For example, putting gloves on to smash a car window would not suffice, but the throwing of a brick at the window would.
A constable's arrest power is not limited to indictable offences, and conditions different from the above apply.
However, a citizen's arrest cannot be made:
inside a polling station, on a person who commits or is suspected of committing an offence of personation under article 30 of the National Assembly for Wales (Representation of the People) Order 2007,[24]
inside a polling station, on a person who commits or is suspected of committing an offence of personation under section 60 of the Representation of the People Act 1983[25]
in relation to an offence of stirring up racial hatred under Part 3 or stirring up religious hatred under Part 3A of the Public Order Act 1986.[26]
In addition to the above, a private person may be authorised to execute an arrest warrant, if the court issuing the warrant has given them the authority to do so.
[edit] Other powersA person may arrest an individual/individuals to prevent an occurring, repeated or a breach of the peace about to occur. This offence definition and power of arrest are contained under the common law definition of 'breach of the peace'.