Best way to repair flagstone cellar floor Brickwork, drystone walling, render are a few subjects covered here. Click here to post questions and get answers

[HomeRenovator]

I'm renovating an old (1850s) stone terrace that has a small coal cellar. I want to use it for storage so plan to install a window into the old coal chute and tank it out with tanking slurry. Before I start though, the floor needs fixing. It's currently laid with old flagstones, some of which have shattered and sunk leaving quite a deep dip in the middle of the floor (see pictures). I'm assuming that the best thing to do is going to be to dig out the flagstones, level with sand and pour a new concrete floor but any other ideas?

Also, is this a job that I need to notify Building Control about?

Many thanks!