A competent person is viewed in case law (Skibs v. Gibson 1964) as:
1. someone who knows what they are looking for,
2. can recognise it when they see it, and
3. (although not mentioned in the case) then knows what to do with it
In short summary, competence has been defined as having the possession of sufficient knowledge, experience and skill to enable the person to know what he or she is doing and to be able to carry out a task in the way in which a person competent in the activity would expect it to be done and to have an appreciation of one’s own limitations.
The CPS have also provided a ‘Definition of Competency’, which is as follows:
A witness is competent if he or she can lawfully be called to give evidence. The principle is set out in Section 53(1) of the Youth Justice and Criminal Evidence Act 1999 (YJCEA 1999): “At every stage in criminal proceedings all persons are (whatever their age) competent to give evidence”.
There are two exceptions:
1) A person is not competent to give evidence in criminal proceedings if it appears to the Court that they are unable to understand questions put to them as a witness and give answers to them which can be understood [Section 53(3) of the YJCEA 1999].
2) A person charged in criminal proceedings is not competent to give evidence in the proceedings for the prosecution (whether he is the only person, or is one of two or more persons, charged in the proceedings) (Section 53 (4) of the YJCEA 1999). A co-accused can only give evidence for the prosecution once he or she ceases to be a co-accused (for example, following a guilty plea).
Lord Justice Bingham said: “The competent person should command the corpus of knowledge which forms part of the professional equipment of the ordinary member of his profession; the law does not require him to be a paragon, combining the qualities of a polymath and prophet” (Eckersley and Other v. Binnie and Partners)
Hope that helps.