Thanks Johna... Yes, excellent feedback and advice that's really appreciated. Has got me to refine my plan and I'll run it by ya...
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The marked 4" in marker is where I had originally planned to screw this wooden cover back. Marked where I can actually reach some brickwork behind it.
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Top (I've shoved the ruler as far as I can).
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Middle (the ruler is sitting flush next to the brickwork).
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Bottom (don't need the ruler as this is where the frame is already screwed into them facing bricks).
Behind that wooden 180mm wide facing piece I'll screw in a couole of battons/blocks. Then once I've screwed this facing width over the gap (as seen in first pic) I'll use the block/blocks behind it - that will sit in the hollow behind - to then drill the left side of the door frame. I think I realistically only have the potential for one screw - at the widest hollow point at the top. Rest of the frame inner is taken up by metal locking mechanism...
Tell me if you think that might be daft. If I don't hear back, I'll assume it's the best option and go ahead then seal it all up... maybe make the inside look a bit prettier first...
Much appreciated, once again. Thanks all...