I've no experience of this but I would tend to agree with the Detail book. The EPDM will seal in the moisture that diffuses through the construction. Using Intello will mean that it is not "breathable", in a the classical sense, but should help to reduce risk (it is simply more diffusion open that would otherwise be the case with other vapour retarders). Moisture migration will tend to occur in the winter and reverse diffusion, back through the Intello, during the summer. In my view inter-seasonal storage of moisture really calls for a WUFI calc (does your Warmcell have sufficient capacity to absorb all the moisture?). Ultimately you'd need fairly robust weather data (and this may be harder to get in the the urban heat island of Wolverhampton).