Thanks all, I would have posted a further amended photo this morning, taking up some of Paul's advice, but the work laptop refused to upload it!
I take on board all the very helpful advice, especially from Neil, Paul and Steve. To that end, I've got the 'Northlight' part-relief structure out, and would have cut the gable ends off it tonight, if I hadn't been 'dragooned' into getting started on the Christmas cards!!
I think that what I'll try first is a modification of the existing Northlight piece, with a representation of sloping roof in (very) low relief, but going the same length up to the road. I'm loathe to scrap the building on the other side of the road, but I take Steve's point and won't rule out replacing it, if something else really looks better.
I will scratchbuild a pair of heavy wooden gates to go across the roadway - something I had been resisting doing for a long time, but I can't find a better way of disguising the end of the layout, and I guess that there's no reason that Callow Lane can't run up to a factory entrance, rather than disappearing elsewhere into the surrounding district.
If I'm still not happy with that, I'll probably try shortening the modified factory by one window 'bay', or two at most, and replacing with a plain, slightly lower, brick wall. I'll also look out for some suitable 'factory scenes' that I can print off and position behind the low relief structure, with just the top bits visible, to give the depth we've mentioned.
I do like the idea of a bit of greenery behind the brick wall, and I'll try that, if I end up with a section of wall, but I have to stress that there really isn't much room there to play with...!
Does anyone have any good sources of photos of 'heavy wooden factory gates', by the way, please?!