Can any one advise on how I can lower the preasure on a Worcester24i ?

Can anyone please advise on how to lower the water preasure on a Worcester 24i junior boiler, at the moment the water preasure is about , but more worrying is the constnt stream of water out of the overflow pipe. So far I have tried truning the fill locking key to looked and letting out water via a radiator but that appears to have no effect. only time over flow stops is when I turn the water off at the mains