I glad i read from the start. Was going to ask if you got permission from the M.O.D to demolish that bunker. Had issues when had my extension built. Tarmac patio with 4 to 5 inch york stones under that. Cleared that lot away ready for the builder. Mini digger arrived to start digging down. About 3 inch of soil then thick sticky clay. lol The digger guy was on his own. After about half an hour he gave up. Call the builder for help and they had to scope a bucket full out and the labourer then had to scrap the clay out the bucket. Was a right mess and didn't get any easier. Down to 800mm and asked inspector if ok to go to 900mm and he said no. Which in fairness is right but the clay is like solid mass and with street on a hill water runs off so on ponding happen.