Deleting Phone Book entries

This seems like a basic question, but today, even the BMW helpdesk have
not been able to help me find an answer!

The problem I have is that on my new 525, I cannot seem to delete
entries from the phone book. Therefore, if I amend a number on my phone
(Sony T610) - which is not uncommon - when the phone is next connected
to the car via Bluetooth, the new number is transferred to the car,
additionally to the old.

For instance, if the name remains unchanged, i.e. Tom, the in-car
display shows Tom with the old number and then Tom1 with the new
number! Neither I not the BMW helpdesk can find a way of deleting the
old number from the car - though they are trying to get back to me next
week!

I am thinking, if I sold the car, would the next owner get all of my
numbers as well as his or her own?

Any ideas?