Hi BOAC,
The closest I can find is on page 108/109 of
http://www.easa.europa.eu/agency-mea...Regulation.pdf
"(c) Where the RVR is not available, RVR values may be derived by converting the reported visibility."
It basically says that if RVR is available - then use it, if it is not available then you may convert the VIS to RVR.