"...if the evidence or question— (a) relates to any evidence adduced by the prosecution about any sexual behaviour of the complainant; and (b) in the opinion of the court, would go no further than is necessary to enable the evidence adduced by the prosecution to be rebutted or explained by or on behalf of the accused."
"(4)For the purposes of subsection (3) no evidence or question shall be regarded as relating to a relevant issue in the case if it appears to the court to be reasonable to assume that the purpose (or main purpose) for which it would be adduced or asked is to establish or elicit material for impugning the credibility of the complainant as a witness."