While looking for something else I came across this rather nice photo of Big Bertha in the York NRM Photo Archive http://www.nrm.org.uk/ourcollection/photo?group=derby&item=314 The photo was taken in 1920 when BB was in MR livery. If you use the Prev link towards the bottom of the page there are two photos of the headlight trial, one of which I have posted before.