Well, for the next project I have decided to play safe and used milliput.
The main core of the building is from a disc of expanded polystyrene, covered in paper so it has some durability whilst being worked (and because I wasn't sure what I was going to do and it's easier, and less destructive) to glue on paper than the polystyrene).
I smoothed on a thin layer of milliput, pressed pieces of actual stone into to get some texture then used a cocktail stick to inscribe the building stones. Pretty simple and straightforward, really.
Here's work so far. The top part is going to be some kind of shrine with an inscribed pattern like some kind of magic circle. Still not sure exactly what it will look like. I have some Ainsty broken columns that will decorate it as well.
I've another building to do after this and I'm not sure what and how I'm going to do it. I'd like to expand my horizons so may well look to some of the other ideas recommended here for creating textures.