I've found it. It's just east of the A1, a little north of Catchpole, between where the A1 crosses the B6326 and the railway mainline.
What interests me is how wrong I was about the graffiti. There's nothing political here; the markings are the usual mix of insults, tags and names. (Given the prevailing news stories it's not surprising I jumped to the conclusion the paint was anti-war.)
I'm disappointed - a unique canvas for creative communication used in a very pedestrian way - but it's good to know I didn't dream the whole thing up.
All contents copyright © 1999-2010 Paul and Emma Bennett