Under CPR 32.2 the general rule is that any fact which needs to be proved by the evidence of witnesses is to be proved:
The court will usually give directions for the mutual exchange of witness statements.
Often the parties will require their cases to be proven, in some material way, by an expert witness. The court, for smaller cases, will usually give permission for the parties to instruct a “single joint expert” (ie instructed jointly by both parties, where the cost is borne equally between the parties). In more complicated cases, the court may give permission for the parties to call their own expert witnesses. Such evidence is usually given by a report in writing, which is mutually exchanged. The court will often direct, at a case management hearing, that the experts meet and seek to narrow the issues, and produce a joint report setting out the issues upon which they can agree, and those where they cannot agree issues.