How to seal a cracked concrete slab roof?

[Dadam]

Bitumen mastic? It’s a rubber crack filler and waterproof.
Round my area you often see it between joins on a path where it has been cut and lifted for telephone lines etc.

That's the kind of stuff I used first time https://www.thompsonsweatherproofing.co.uk/roofing/emergency-roof-repair-mastic/
I actually found the old tin in my garage! Probably set solid now but I bought a new tin anyway.

I chipped off as much of the old mastic and cleaned up as much as possible, then slathered a load of this new tin across the crack and 4" either side, continuing round the edge where it meets the house wall, and the other side down the edge. We've had a fair bit of rain since and nothing's come in.