
Bummer, I'm afraid, Tiki. I've scoured my Old Portsmouth stuff. Images. Maps. Everything I have to hand. Can't even find King William Place on the 1896 OS.
Portsmouth Central Library holds many old OS maps. It even has a dedicated Portsmouth Room, as I remember. Have ye tried them?
See, as far as I've got here; The Longstore location is simply lost to us. I've studied arial shots of the area we'd be talking. And, as far back as they could
get such a shot ~ or from ground shots of other features around there? Waste ground.
Ye have to realise, see; Many of these " Barracks " were actually little more than rat and disease infested shanty towns. They were razed as 'real' places became available.
I saw a mention of Clarence Barracks bordering KWP? Great, I thought. Search for an image of that then. Found " The
New Clarence Barracks " See? And that place looked 1800's! The original CB then was probably, as I say, little more than a shanty. And who would want to preserve images of That sort of thing?
Rather pathetic little 'best I can do' is a monotone water colour (Eh?!

) It depicts a scene of some 'period' looking soldiers, doing something and incorporating a distant view of the area in question.
But, all it really gives us is a piece of the church and the roof tops around it. One of those roof tops might have been the barracks .....

Naah. Bit bloody tenuous, isn't it?
I've got my partner in crime, " Pompey Boy " on this case too. But, having read ye initial posting on here, he's come to me with, like; "

WTF?!
Where?!? ". Somewhere vaguely to the south of the church would now be my own, better educated than this morning, guess. But, yeah; Slum land, in those days. Hardly an inspiration to artists then.