How to dig a trench for a cable for a garden office

I'm trying to dig a trench for the electrical cable for a shed/office I'm building in my back garden. This is about 7 m long and should be 0.5 m deep. My soil seems to be about 5 cm of soil/turf and then soil which is full of gravel, stones and building waste like bricks, etc. What is the best way to approach this?

I tried first just making cuts into the soil using a spade to make a narrow slot. That didn't work once I got to the stones.

I also tried using an SDS drill with a 600 mm long chisel bit, but that tends to jam in between the stones and gravel, so I spent a lot of time trying to dislodge it from the ground.

The third technique was with the pressure washer and wet vac, to blast away the soil and suck up the water and smaller stones. This seems to be doing something, but it is painstakingly slow. Since the slot is quite narrow it's pretty difficult to pull up the stones and gravel.

Is there some other piece of equipment I should consider using, or some other technique?