Hey Kate,
Well I have exactly the same problem as you. Same car, year etc etc. As soon as I hit 70mph the car goes into limp mode. Can be very frustrating when you hit a hill and go down to 65mph and have the lorries you just over took come and over take you.
I had the ECU read, inlet manifold pressure error.
My garage wants to replace the sensor for that (at a cost of £120). But I will have a look at the hose, anyone know where abouts it is? left/right of the engine?