The CPS site has different guidance.
"The proceedings can be commenced when both the following apply:
at any time within 2 years from the date of the offence to which the proceedings relate, and
within six months from the first date the victim made a statement or was interviewed about the incident, (section 39A(4) CJA 1988)."
That means someone could take months to report an incident and subsequently be interviewed but the six months' limit would not be backdated.