you can check the condition of the multi v belt that drives the pas pump, if the diesel is similar to the petrol these are tensioned with a spring loaded pulley, normally a worn incorrectly tensioned belt would squeel like mad at all sorts of revs, it is deffo worth checking this along with your power steering fluid for a start, if these check out ok then its either the rack or the pump

which wont be cheap, so lets hope it one of the 1st things!!