"Is there usually some sort of a scale on the horizon in a/c fitted with a FPV to give an indication of drift? How accurate is it if (i.e every degree, two degrees etc) so and what value would it extend to?"
On the 777, no - I don't know about other types.
I have never seen an azimuth scale on an AI, though I believe the drift representation is linear. I haven't found what 'full-scale deflection' is from the manuals but the FPV was starting to go off the edge of the display in a 200Kt crosswind the other day!