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Hello @sausage
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about what to fill the gap between your walls.
The curved section appears to be old tiles painted over. I propose grinding the surface of the tiles just to remove the glaze on its surface. I then suggest cleaning the gap as best as possible, making sure that all the dust and debris are removed. Once the area is clean, I suggest using Lanko 5kg 136 Rapid Patching Mortar to fill the gap.
If you have any other questions we can help with, please let us know.
Eric
