[erockrph]

This is a highly regarded and much discussed beer here in England.
The brewery give little to no information on recipes, but the  general consensus is that the beer is colour adjusted with brewers caramel.
Like jeffy, I used to use a little black malt to adjust the colour but no longer bother these days when I make it.

This ^^^^^

The Brits have been using "brewer's caramel" for artificial coloring for more than 100 years.  It's like a black tar and they add just a little bit.  You can simulate the same thing at home just by adding a couple ounces of dehusked black malt.

The effect of a longer boil is very very small.  You'd have to boil for far longer to get significant darkening, or else concentrate your boil volume and then add water at the end.  Commercial brewers probably don't do those things, but who knows.

+1 - A recipe I have for Old Peculier that is allegedly from the brewery at Theakston calls for caramel color as well. It's pretty much the same thing that colors things like cola. http://www.ddwcolor.com/colorant/caramel-color/