I just put back axle for easy explanation.
It is the rear subframe that the trailing arms connect to, subframe and trailing arms are badly rusted, ok for this MOT with a advisory, but won't pass next years MOT after a winter of salt and wet weather.
To replace the subframe and trailing arms he quoted £950 , that is a long as they do not hit any problems while doing the work, so he said budget for £1000+ for the work.