Thanks folks. Good points made/asked.
I’m not wanting a drystone look. The plan (and plans change!) is to infill the major gaps during the finessing stage. I’ll probably use a paste of the plaster and some dilute PVA. As to how heavily overgrown it will be ..... I don’t yet know. I’m not planning for any trees inside it (space is tight as it is) but there’ll be tufts in walls, shrubs and maybe some ivy. Debris will be limited, on the basis that it’s been robbed out.
The most recent change of plan is to lose the back wall to damage, as including it would require too much height for a relatively small space. The main feature is intended to be the big arch and it’ll stand out more without the wall behind it.
I’ve built up the lower sections of the walls that will support the main arch, and so the front section is now really taking shape.
Any joints that are off can also be corrected during finessing, as can over large blocks (which will be cut back).
NB: As I’m going to drill out the windows, I’ll have to add some plaster afterwards to turn blocks into lintels, etc.