Ok, finally I managed to talk to one of the local builders who was involed in putting in the original septic tank and soakaway, and who has added to the soakaway recently.
He says that the pipe from the tank to the soakaway runs along the edge of the road and that the yard drain is not cut into it. Also that he and my ex extended the soakaway a couple of years ago and apart form the problem that it is in a bog (the entire dale is a bog!) it is working fine.
So we think that the yard drain, which is ancient, must also go into a soakaway under the byre, which has got silted up. So we are going to dig an additional channel so that, in heavy rain, when the drain soakaway can't cope, the excess can run into the beck. That's not a problem, since it is only rain.
I am so relieved 1. My house drainage system is fine and 2. There is a simple solution to the yard drainage