Patio dpc

I am currently planning a new patio and cannot determine where the DPC is. My extension was built in approx 2000. My feeling is the DPC is likely to be inline with the bell cast render but I cannot find any signs of a DPC where I have chipped away some render.

I want to run a patio step the full width of the rear of the house and ideally wouldn't go any lower than the bell cast due to height of step out being too high. How can I determine the DPC and if it's at the bell cast how I protect against bridging. Would running an ACO drain below the bell cast solve the problem?

From the step I would then run the remainder of the patio. The air brick would be extended to the front of the step.

2000 years ago?

The year 2000

I think I've found the DPC below the bell cast.

Because the height of the first step from house to patio would be too great if I went 150mm below the DPC what options do I have to laying the patio on the DPC line?

Thanks