It is very rare to be able to obtain the name of an informant.
A whole body of case law has been built up over the last 100 years and the general view of the law is that the police force cannot act effectively without a whole body of informants. We know they are snitches, snouts and grasses and they get paid and all other sorts of things. But that is not enough, in itself, to have that evidence ruled inadmissable.
Sometimes a Court might say that an informant is simply unreliable because of incentives offered by the police although this does not happen very often.
But you will see from below a very important qualification in section 67 (2).
Recently, our whole body of law in relation to evidence was codified into the Evidence Act 2006 and we set out the following new sections from that Act.
Section 64 - Informers
(1) An informer has a privilege in respect of information that would disclose, or is likely to disclose, the informer's identity
(2) A person is an informer for the purposes of this section if the person -
a) has supplied, gratuitously or for reward, information to an enforcement agency, or to a representative of an enforcement agency, concerning the possible or actual commission of an offence in circumstances in which the person has a reasonable expectation that his or her identity will not be disclosed; and
(b) is not called as a witness by the prosecution to give evidence relating to that information.
(3) An informer may be a member of the police working undercover.