Not sure how I forgot this, since I drank it three weeks ago -

Grainstore Brewery Rutland Panther, 3.8%
According to the beer festival programme, "Black brew with drinkability of mild or light bitter, combined with roast flavours associated with stout."

According to me, that's pretty much what it was - a light, even slightly thin texture and not too much alcohol, but with the look and taste of stout. Could catch on.