It works like this, Simon:
If a witness makes a statement to the police and a prosecution follows the prosecution (usually the CPS) will show that statement to the defence as part of their “disclosure“ before the trial. The defence has the right to request that the witness attends court and gives evidence in person. This means that they can then be cross examined by the defence (that is, the defence can question them , put alternative scenarios to them, generally challenge their version of events). If the witness does not attend court their statement cannot be used in evidence without the agreement of the defence (which is unlikely to be forthcoming if the evidence is disputed).
A similar process operates the other way round and if the defence wants to use evidence provided by a witness who has made a statement supporting the defendant’s version of events they too must attend court or the evidence provided in the statement cannot be used.
All statements (for both sides) form evidence. But they can only be used in isolation (i.e. without the witness attending court to give evidence in person) if the opposing side agrees.