Might someone shed some light on the 'witness' referred to in schedule 1 part 3?

3.—(1) For the purposes of this Schedule, a person gives informed consent to take part, or that a subject is to take part, in a clinical trial only if his decision—
(a) is given freely after that person is informed of the nature, significance, implications and risks of the trial; and
(b) either—
(i) is evidenced in writing, dated and signed, or otherwise marked, by that person so as to indicate his consent, or
(ii) if the person is unable to sign or to mark a document so as to indicate his consent, is given orally in the presence of at least one witness and recorded in writing.

The presence of this provision indicates that there may be cases when a subject, not considered to be incapacitated, is unable to mark a document but clearly able to express informed consent. In such cases, who might the witness be? I understand the witness is not a legal representative, so are there any restrictions on who that witness might be? May they be a member of the research team for example?