Reset BT Freestyle 750 base unit ?

Hello .. Is it possible to reset a BT Freestyle 750 base unit to have NO phone registered ( ie..delete the existing factory installed memory ) . I have a twin handset system . One of the handsets got permanently damaged in water , it's now in the bin , so I bought a new one . It registered ok at the base station , but only as handset no 3 . Handset No 1 cannot be deleted from the base station memory no matter how much I try . Now my base station has three phones in it's memory . How can I delete the damaged handset no 1 permanently ?