www.bia.org

Thanks for the above link - very informative. Also point taken about the gap not being for insulation, but as a way to stop damp migrating through the complete wall. After all cavity walls were used for very many before heat loss or conservation became an issue.

You would perhaps agree however that having a cavity wall does allow for having 'cavity wall insulation' added either during construction (expanded polystyrene) or subsequently (as fibre for example).

Given that the porch will be double glazed etc I think I will choose cavity for the above reason. Thanks for the help it has brought me to a decision.