I believe a renumber is now free (it used to be �35.84 inc VAT, but free for nuisance calls sufferers!). Phone 150 and go through to customer services. (I don't work for BT anymore, but if they mention a charge, insist it's because you're receiving nuisance calls!)
Another option would be "BT Call Sign" where you get given a second phone number but it has a different ring to your original one. You could give your friends and family the new number with the different ring so you could then distinguish between "friend and foe"