I'd guess at cement. It goes a few inches back so I don't really want to go down the rabbit hole of starting to scrape it out and going further and further back and ending up with a massive hole.
go down the rabbit hole
DIY is one big rabbit hole
@NotThamesWater started
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I'd guess at cement. It goes a few inches back so I don't really want to go down the rabbit hole of starting to scrape it out and going further and further back and ending up with a massive hole.