To find the DPC you need to use, say, a flat bladed screwdriver to to start scraping out a little mortar from the beds. Start at ground level and work up bed by bed.
Any DPC would probably be a slate DPC although other materials were used.
Your air brick is very low - maybe the ground level has been raised over the years?
Presumably, you have a suspended floor - your joist tails might be at risk of rotting?
Is there damp damage showing on the inside of the solid wall?
Thanks for the responses, internally the walls are grand, can't see any signs of damp. The reason I ask is I'd like to lay some external porcelain tiles to tidy the area up. Told I need 150 mm clearance from the DPC to make sure I don't get raising damp. I guess as the air brick is so low it's best to leave as is.
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