An expert's opinion & comment.

Stumbled upon this thread, don't normally write reviews. I paint lots of exteriors, both smooth and roughcast, as a professional decorator (since 1988).
My advice would be to buy the trade (rather than the retail) version. It is an excellent product which goes on easily and lasts well. Any issues with flaking will be due to insufficient or incorrect preparation. Any issues with coverage will be due to the nature of the surface. The guidelines on the tin are just that, guidelines. On heavy roughcast you will not get more than about 4 or 5 m2 per litre.
Although I guess Sandtex sell lots of the retail stuff, it's not as good as the trade paint, which has more 'body' (thicker), more pigment (I suspect) and a better formulation (more resin?, more fungicide?). This means it can stand more thinning for the first coat if this is required (not always). Make sure you prep the surface properly. Don't use stabiliser unless the surface is actually chalking. Dulux and Johnstones (trade) are also good.