The Building Inspector has had a look at the sewer manhole that unexpectedly appeared under my patio. He has given 3 options.
Make the extension smaller and keep the manhole outside. A non starter, the extension would be so small, it wouldn't be worth doing.
Dig down to the depth of the sewer, bridge the pipes and fill with concrete. They would have to be dug by hand and the concrete costs would be too great.
Sink mini piles around the sewer and build on that.
Having spoken to the architect, he says that will cost a similar amount to the traditional method of building foundations, but it will have to be carried out by a specialist. So everything stops while a specialist is sought, they have a look at the job, give a price, then assuming it is accepted, slots us in to their no doubt full diary and then once done, the builders can carry on.
In the meantime, we wait.
They could always make a start on the porch on the front of the house.