Lynda, our neighbour, hired a mini digger and I was given a chance to use it for the afternoon. The hoops are 1.5m apart, and the digger was able to squeeze between them. I did manage to knock into one of the door uprights and dislodge one of the foundation pipes. Compared to digging two trenches 12m/40ft by hand, its a reasonable tradeoff for my poor operating skills.
The trench will be used to lay the plastic cover of the cold frame, and soil will be placed over it to keep it in place. A few you tube videos have given me an idea of what is required.