Thanks again for all the info. I've found this 1979 picture:Bell Lane signalbox, Leicester 1979
© Copyright Peter Whatley
and licensed for reuse under this Creative Commons Licence
.(Click on picture to go to the larger original.)
I note that this box also sits on an embankment. The rear landing at the top of the stairs can just be seen, so the one on my model is the reverse way round.
I'll need to redo the windows of the model, I think - the glazing bars across the windows are rather oversize, as well as the sections of glazing not overlapping as in real life. And thin down the finials a bit as well!