Went and balanced 215/50 17" wheels - no more vibration at high speed. I have tried 40psi fronts with 37psi rear but it seems the back is a bit floaty when pushing through corners while the front is not as crisp on turn in. I know the Mondeo is a relatively heavy car and maybe i'm nitpicking. I will continue experimenting with tyre pressures, i will put a bit more all round.
Interestingly, the bloke who balanced the wheels suggested i try 36 psi all round......sounds a bit soft!
