Sorry to hear about the problem Kim.
I think if you take it to a good garage who give high priority to customer care you should be able to get it sorted.
I live right at the southern end of the country and the local independent garage I use is Subaru4U.
I had problems with a noise occurring at the front end when the car went above 50mph. They drove it and listened and whilst they could hear it couldn't immediately establish what it was. So they suspended the car on the ramps in their workshop one guy sat inside and got it up to 50 mph and another mechanic went round all the bearings with his listening kit (sorry don't know the name!). They then switched over and repeated the exercise. The first mechanic didn't reveal his thoughts so they could see independently what they concluded. Afterwards both agreed everything seemed okay except the front drivers side wheel bearing sounded noisier than all the other wheel bearings. When they stripped it down they found that it hadn't failed but there was some slight wear to one side.They replaced it and the noise stopped.
If they hadn't been able to detect the problem that way they tell me they are able to place sensors around the car near where the problem is (bushes, bearings etc) and they then have a bit of kit for listening to the noise inside the car whilst driving. This can help them narrow down what the problem is more easily.
In terms of customer service I couldn't fault the effort they went to. Maybe you could check with other well regarded garages near you what extent of diagnosis they are prepared to carry out.
I hope that helps and also gives you hope. There's nothing worse than having an irritating noise on the car and that feeling it is never going to get sorted.
L
PS I hope you have checked your tyre pressures- I rightly got a telling off from The Swede recently. If the pressures are low it will make the suspension noisier!