Pete recently did an interview on BBC Radio 4 - You and Yours - which investigated a case history of failed cavity wall insulation.
There are streets of these lovely old houses in Birmingham. We have been fortunate enough to work on numerous such examples - these are a couple of houses in which we have restored the porticos and stucco work to the facades. One of the biggest problems with these houses is that over the years, lime render has been replaced with concrete. The houses then become very wet and suffer from all the symptoms of 'rising damp' - as soon as we strip the hollow sounding cement render and replace it with lime, the buildings dry out and all the dampness internally disappears. You should NEVER use cement reder on an old house. They should always be painted with breathable paints - clay or limewash - NEVER use plastic emulsion masonry paints.
Repairs to porticos like these need not be extravagantly expensive. Usually there are sufficient examples of the original mouldings to allow us to take moulds and re-construct the detail. Fluted columns are often badly worn - these can be repaired and re-shaped. Lead flashing often deteriorates and allows water to penetrate - again, simple to repair with the right expertise.