I've just done mine !
Use a paint stripping tool to get the 'big bits' off..........I then peeled the paint off to an acceptable point, i.e. joint in stonework, change of direction, etc. Then a good stiff wire brush to take care of the flaky bits.
I repainted them using a 'Pliolite' brilliant white paint. This is an oil-based masonry paint and in addition to covering very well and looking very smart, any future dirt can just be blasted off with a pressure washer as it will 'sit' on the surface instead of permeating through....................or at least that's what I was told :o)