Leaking sewer pipe outside house?

Hi, my toilet has just stopped emptying when flushed - the water drains away very slowly so I put the pressure washer in and blasted it, and now I have a puddle outside the house! I've lifted the flags and there's a sort of terracotta pipe just under them, and water is all around it which bubbles when you flush the toilet so I clearly have a leak, and probably earth has got in and blocked it. There's no manhole, unlike in my neighbour's garden, just a pipe which comes out of the house and runs under the flags to the pavement. Do I need to replace all this pipe? Any idea how much it costs, or is the water-board responsible?

Thanks